diff --git a/include/cantera/base/XML_Writer.h b/include/cantera/base/XML_Writer.h
index f57edf5e3..cda87fa2e 100644
--- a/include/cantera/base/XML_Writer.h
+++ b/include/cantera/base/XML_Writer.h
@@ -58,7 +58,7 @@ public:
         s << "<!--" << comment << "-->" << std::endl;
     }
 
-    inline void XML_open(std::ostream& s, const std::string& tag, const std::string p = "") {
+    inline void XML_open(std::ostream& s, const std::string& tag, const std::string& p = "") {
         for (int n = 0; n < _level; n++) {
             s << _indent;
         }
diff --git a/include/cantera/base/ctml.h b/include/cantera/base/ctml.h
index a602008f6..5dac32542 100644
--- a/include/cantera/base/ctml.h
+++ b/include/cantera/base/ctml.h
@@ -73,8 +73,8 @@ extern std::string INT_Format;
  *       typeString should be codified as to its usage.
  */
 void addInteger(Cantera::XML_Node& node, const std::string& titleString,
-                const int value, const std::string unitsString="",
-                const std::string typeString="");
+                const int value, const std::string& unitsString="",
+                const std::string& typeString="");
 
 //!  This function adds a child node with the name, "float", with a value
 //!  consisting of a single floating point number
@@ -125,8 +125,8 @@ void addInteger(Cantera::XML_Node& node, const std::string& titleString,
  *       typeString should be codified as to its usage.
  */
 void addFloat(Cantera::XML_Node& node, const std::string& titleString,
-              const doublereal value, const std::string unitsString="",
-              const std::string typeString="", const doublereal minval = Cantera::Undef,
+              const doublereal value, const std::string& unitsString="",
+              const std::string& typeString="", const doublereal minval = Cantera::Undef,
               const doublereal maxval = Cantera::Undef);
 
 //!  This function adds a child node with the name, "floatArray", with a value
@@ -183,7 +183,7 @@ void addFloat(Cantera::XML_Node& node, const std::string& titleString,
  */
 void addFloatArray(Cantera::XML_Node& node,  const std::string& titleString,
                    const size_t n,  const doublereal* const values,
-                   const std::string unitsString="", const std::string typeString="",
+                   const std::string& unitsString="", const std::string& typeString="",
                    const doublereal minval = Cantera::Undef,
                    const doublereal maxval = Cantera::Undef);
 
@@ -216,7 +216,7 @@ void addFloatArray(Cantera::XML_Node& node,  const std::string& titleString,
  *   @param typeString    String type. This is an optional parameter.
  */
 void addString(Cantera::XML_Node& node,  const std::string& titleString,
-               const std::string& valueString, const std::string typeString="");
+               const std::string& valueString, const std::string& typeString="");
 
 
 //!  This function reads the current node or a  child node of the current node
@@ -276,8 +276,8 @@ void addString(Cantera::XML_Node& node,  const std::string& titleString,
  *   @return              Returns the number of floats read into v.
  */
 size_t getFloatArray(const Cantera::XML_Node& node, std::vector<doublereal> & v,
-                     const bool convert=true, const std::string unitsString="",
-                     const std::string nodeName = "floatArray");
+                     const bool convert=true, const std::string& unitsString="",
+                     const std::string& nodeName = "floatArray");
 
 //! This function interprets the value portion of an XML element
 //! as a string. It then separates the string up into tokens
@@ -472,7 +472,7 @@ void getIntegers(const Cantera::XML_Node& node, std::map<std::string,int>& v);
  *                 which implies that no conversion is allowed.
  */
 doublereal getFloat(const Cantera::XML_Node& parent, const std::string& name,
-                    const std::string type="");
+                    const std::string& type="");
 
 //!  Get a floating-point value from the current XML element
 /*!
@@ -506,7 +506,7 @@ doublereal getFloat(const Cantera::XML_Node& parent, const std::string& name,
  *                 and "" , for no conversion. The default value is "",
  *                 which implies that no conversion is allowed.
  */
-doublereal getFloatCurrent(const Cantera::XML_Node& currXML, const std::string type="");
+doublereal getFloatCurrent(const Cantera::XML_Node& currXML, const std::string& type="");
 
 //!  Get an optional floating-point value from a child element.
 /*!
@@ -545,7 +545,7 @@ doublereal getFloatCurrent(const Cantera::XML_Node& currXML, const std::string t
  * @return returns true if the child element named "name" exists
  */
 bool getOptionalFloat(const Cantera::XML_Node& parent, const std::string& name,
-                      doublereal& fltRtn, const std::string type="");
+                      doublereal& fltRtn, const std::string& type="");
 
 
 //!  Get a vector of floating-point values from a child element.
@@ -689,7 +689,7 @@ doublereal getFloatDefaultUnits(const Cantera::XML_Node& parent,
  *
  *   @return True if the nodeName XML node exists. False otherwise.
  */
-bool getOptionalModel(const Cantera::XML_Node& parent, const std::string nodeName,
+bool getOptionalModel(const Cantera::XML_Node& parent, const std::string& nodeName,
                       std::string& modelName);
 
 //! Search the child nodes of the current node for an XML Node with a Title
@@ -789,13 +789,13 @@ std::string getChildValue(const Cantera::XML_Node& parent,
  *  @param file    Name of the file
  *  @param debug   Turn on debugging printing
  */
-void get_CTML_Tree(Cantera::XML_Node* node, const std::string file,
+void get_CTML_Tree(Cantera::XML_Node* node, const std::string& file,
                    const int debug = 0);
 
 //! Read an ctml file from a file and fill up an XML tree.
 //!   @param file    Name of the file
 //!   @return        Root of the tree
-Cantera::XML_Node getCtmlTree(const std::string file);
+Cantera::XML_Node getCtmlTree(const std::string& file);
 
 //! Convert a cti file into a ctml file
 /*!
diff --git a/include/cantera/base/xml.h b/include/cantera/base/xml.h
index 6cc6e415f..2448f1560 100644
--- a/include/cantera/base/xml.h
+++ b/include/cantera/base/xml.h
@@ -147,7 +147,7 @@ public:
      *  @param parent   Pointer to the parent for this node in the tree.
      *                  A value of 0 indicates this is the top of the tree.
      */
-    XML_Node(const std::string nm, XML_Node* const parent);
+    XML_Node(const std::string& nm, XML_Node* const parent);
 
     //! Copy constructor
     /*!
@@ -247,7 +247,7 @@ public:
      *   @return  Returns a reference to the created child XML_Node object
      */
     XML_Node& addChild(const std::string& name, const doublereal value,
-                       const std::string fmt="%g");
+                       const std::string& fmt="%g");
 
     //! Remove a child from this node's list of children
     /*!
@@ -275,7 +275,7 @@ public:
      * @param fmt  Format of the printf string conversion of the double.
      *             Default is "%g". Must be less than 63 chars
      */
-    void addValue(const doublereal val, const std::string fmt="%g");
+    void addValue(const doublereal val, const std::string& fmt="%g");
 
     //! Return the value of an XML node as a string
     /*!
@@ -339,7 +339,7 @@ public:
      *                Default is "%g".
      */
     void addAttribute(const std::string& attrib, const doublereal value,
-                      const std::string fmt="%g");
+                      const std::string& fmt="%g");
 
     //! The operator[] is overloaded to provide a lookup capability
     //!  on attributes for the current XML element.
diff --git a/src/base/ct2ctml.cpp b/src/base/ct2ctml.cpp
index c14c72973..1a0432eb8 100644
--- a/src/base/ct2ctml.cpp
+++ b/src/base/ct2ctml.cpp
@@ -229,7 +229,7 @@ void ck2cti(const std::string& in_file, const std::string& thermo_file,
  *  @param file    Name of the file
  *  @param debug   Turn on debugging printing
  */
-void get_CTML_Tree(Cantera::XML_Node* rootPtr, const std::string file, const int debug)
+void get_CTML_Tree(Cantera::XML_Node* rootPtr, const std::string& file, const int debug)
 {
     std::string ff, ext = "";
 
@@ -280,7 +280,7 @@ void get_CTML_Tree(Cantera::XML_Node* rootPtr, const std::string file, const int
     fin.close();
 }
 
-Cantera::XML_Node getCtmlTree(const std::string file)
+Cantera::XML_Node getCtmlTree(const std::string& file)
 {
     Cantera::XML_Node root;
     get_CTML_Tree(&root, file);
diff --git a/src/base/ctml.cpp b/src/base/ctml.cpp
index 976409d8c..58702953f 100644
--- a/src/base/ctml.cpp
+++ b/src/base/ctml.cpp
@@ -78,7 +78,7 @@ static doublereal fpValue(std::string val)
  *                        is to have an empty string.
  */
 void addInteger(Cantera::XML_Node& node, const std::string& title, const int val,
-                const std::string units, const std::string type)
+                const std::string& units, const std::string& type)
 {
 #ifdef CTML_VERSION_1_4
     XML_Node& f = node.addChild("integer", val);
@@ -138,8 +138,8 @@ void addInteger(Cantera::XML_Node& node, const std::string& title, const int val
  *       as to its usage.
  */
 void addFloat(Cantera::XML_Node& node, const std::string& title,
-              const doublereal val, const std::string units,
-              const std::string type, const doublereal minval,
+              const doublereal val, const std::string& units,
+              const std::string& type, const doublereal minval,
               const doublereal maxval)
 {
 #ifdef CTML_VERSION_1_4
@@ -214,8 +214,8 @@ void addFloat(Cantera::XML_Node& node, const std::string& title,
  * @todo typeString should be codified as to its usage.
  */
 void addFloatArray(Cantera::XML_Node& node, const std::string& title, const size_t n,
-                   const doublereal* const vals, const std::string units,
-                   const std::string type,
+                   const doublereal* const vals, const std::string& units,
+                   const std::string& type,
                    const doublereal minval, const doublereal maxval)
 {
     size_t i;
@@ -278,7 +278,7 @@ void addFloatArray(Cantera::XML_Node& node, const std::string& title, const size
  */
 void addString(Cantera::XML_Node& node, const std::string& titleString,
                const std::string& valueString,
-               const std::string typeString)
+               const std::string& typeString)
 {
     XML_Node& f = node.addChild("string", valueString);
     f.addAttribute("title", titleString);
@@ -511,7 +511,7 @@ void getFloats(const Cantera::XML_Node& node, std::map<std::string, double>& v,
  */
 doublereal getFloat(const Cantera::XML_Node& parent,
                     const std::string& name,
-                    const std::string type)
+                    const std::string& type)
 {
     if (!parent.hasChild(name))
         throw CanteraError("getFloat (called from XML Node \"" +
@@ -555,7 +555,7 @@ doublereal getFloat(const Cantera::XML_Node& parent,
  *                 which implies that no conversion is allowed.
  */
 doublereal getFloatCurrent(const Cantera::XML_Node& node,
-                           const std::string type)
+                           const std::string& type)
 {
     doublereal x, x0, x1, fctr = 1.0;
     string units, vmin, vmax;
@@ -614,7 +614,7 @@ doublereal getFloatCurrent(const Cantera::XML_Node& node,
 bool getOptionalFloat(const Cantera::XML_Node& parent,
                       const std::string& name,
                       doublereal& fltRtn,
-                      const std::string type)
+                      const std::string& type)
 {
     if (parent.hasChild(name)) {
         fltRtn= getFloat(parent, name, type);
@@ -719,7 +719,7 @@ doublereal getFloatDefaultUnits(const Cantera::XML_Node& parent,
  *
  *   @return True if the nodeName XML node exists. False otherwise.
  */
-bool getOptionalModel(const Cantera::XML_Node& parent, const std::string nodeName,
+bool getOptionalModel(const Cantera::XML_Node& parent, const std::string& nodeName,
                       std::string& modelName)
 {
     if (parent.hasChild(nodeName)) {
@@ -849,8 +849,8 @@ int getInteger(const Cantera::XML_Node& parent, const std::string& name)
  *  @note change the v to a std::vector to eliminate a doxygen error. No idea why doxygen needs this.
  */
 size_t getFloatArray(const Cantera::XML_Node& node, std::vector<doublereal> & v,
-                     const bool convert, const std::string unitsString,
-                     const std::string nodeName)
+                     const bool convert, const std::string& unitsString,
+                     const std::string& nodeName)
 {
     std::string::size_type icom;
     string numstr;
diff --git a/src/base/xml.cpp b/src/base/xml.cpp
index a0c71803c..ebcb9cbda 100644
--- a/src/base/xml.cpp
+++ b/src/base/xml.cpp
@@ -368,7 +368,7 @@ XML_Node::XML_Node(const char* cnm)  :
  *  @param parent   Pointer to the parent for this node in the tree.
  *                  A value of zero 0 indicates this is the top of the tree.
  */
-XML_Node::XML_Node(const std::string nm, XML_Node* const parent) :
+XML_Node::XML_Node(const std::string& nm, XML_Node* const parent) :
     m_name(nm),
     m_value(""),
     m_parent(parent),
@@ -577,7 +577,7 @@ XML_Node& XML_Node::addChild(const std::string& name, const std::string& value)
  *   @return  Returns a reference to the created child XML_Node object
  */
 XML_Node& XML_Node::addChild(const std::string& name, const doublereal value,
-                             const std::string fmt)
+                             const std::string& fmt)
 {
     XML_Node& c = addChild(name);
     c.addValue(value, fmt);
@@ -631,7 +631,7 @@ void XML_Node::addValue(const std::string& val)
  * @param fmt  Format of the printf string conversion of the double.
  *             Default is "%g" Must be less than 63 chars
  */
-void XML_Node::addValue(const doublereal val, const std::string fmt)
+void XML_Node::addValue(const doublereal val, const std::string& fmt)
 {
     m_value = stripws(fp2str(val, fmt));
 }
@@ -719,7 +719,7 @@ void XML_Node::addAttribute(const std::string& attrib, const std::string& value)
  *                Default is "%g".
  */
 void XML_Node::addAttribute(const std::string& attrib,
-                            const doublereal value, const std::string fmt)
+                            const doublereal value, const std::string& fmt)
 {
     m_attribs[attrib] = fp2str(value, fmt);
 }
diff --git a/src/transport/TortuosityBase.cpp b/src/transport/TortuosityBase.cpp
index fecb71e22..0e62845b0 100644
--- a/src/transport/TortuosityBase.cpp
+++ b/src/transport/TortuosityBase.cpp
@@ -18,7 +18,7 @@
 namespace Cantera
 {
 //====================================================================================================================
-static void err(const std::string r)
+static void err(const std::string& r)
 {
     throw Cantera::CanteraError("TortuosityBase", "Error calling base class " + r);
 }