diff --git a/include/cantera/thermo/Phase.h b/include/cantera/thermo/Phase.h
index d6f3a6781..9f9c1d7ea 100644
--- a/include/cantera/thermo/Phase.h
+++ b/include/cantera/thermo/Phase.h
@@ -4,7 +4,7 @@
  */
 
 // This file is part of Cantera. See License.txt in the top-level directory or
-// at http://www.cantera.org/license.txt for license and copyright information.
+// at https://cantera.org/license.txt for license and copyright information.
 
 #ifndef CT_PHASE_H
 #define CT_PHASE_H
@@ -707,6 +707,10 @@ public:
     //! Return the Species object for the named species. Changes to this object
     //! do not affect the ThermoPhase object until the #modifySpecies function
     //! is called.
+    /*!
+     * @deprecated To be removed after Cantera 2.5. Replaceable with
+     * Phase::species(size_t) via Phase::speciesIndex(std::string&).
+     */
     shared_ptr<Species> species(const std::string& name) const;
 
     //! Return the Species object for species whose index is *k*. Changes to
@@ -747,6 +751,17 @@ public:
     //! change in state is detected
     virtual void invalidateCache();
 
+    //! Returns `true` if case sensitive species names are enforced
+    bool caseSensitiveSpecies() const {
+        return m_caseSensitiveSpecies;
+    }
+
+    //! Set flag that determines whether case sensitive species are enforced
+    //! in look-up operations, e.g. speciesIndex
+    void setCaseSensitiveSpecies(bool cflag = true) {
+        m_caseSensitiveSpecies = cflag;
+    }
+
 protected:
     //! Cached for saved calculations within each ThermoPhase.
     /*!
@@ -794,6 +809,14 @@ protected:
     //! Flag determining behavior when adding species with an undefined element
     UndefElement::behavior m_undefinedElementBehavior;
 
+    //! Find lowercase species name in m_speciesIndices when case sensitive
+    //! species names are not enforced. Raise exception if lowercase name
+    //! is not unique.
+    size_t findSpeciesLower(const std::string& nameStr) const;
+
+    //! Flag determining whether case sensitive species names are enforced
+    bool m_caseSensitiveSpecies;
+
 private:
     XML_Node* m_xml; //!< XML node containing the XML info for this phase
 
diff --git a/interfaces/cython/cantera/_cantera.pxd b/interfaces/cython/cantera/_cantera.pxd
index 26eff9b62..af5f9c147 100644
--- a/interfaces/cython/cantera/_cantera.pxd
+++ b/interfaces/cython/cantera/_cantera.pxd
@@ -165,12 +165,13 @@ cdef extern from "cantera/thermo/ThermoPhase.h" namespace "Cantera":
 
         # species properties
         size_t nSpecies()
-        shared_ptr[CxxSpecies] species(string) except +translate_exception
         shared_ptr[CxxSpecies] species(size_t) except +translate_exception
         size_t speciesIndex(string) except +translate_exception
         string speciesName(size_t) except +translate_exception
         double nAtoms(size_t, size_t) except +translate_exception
         void getAtoms(size_t, double*) except +translate_exception
+        cbool caseSensitiveSpecies()
+        void setCaseSensitiveSpecies(cbool)
 
         double molecularWeight(size_t) except +translate_exception
         double meanMolecularWeight()
diff --git a/interfaces/cython/cantera/thermo.pyx b/interfaces/cython/cantera/thermo.pyx
index b8db5d685..f54d6b332 100644
--- a/interfaces/cython/cantera/thermo.pyx
+++ b/interfaces/cython/cantera/thermo.pyx
@@ -1,5 +1,5 @@
 # This file is part of Cantera. See License.txt in the top-level directory or
-# at http://www.cantera.org/license.txt for license and copyright information.
+# at https://cantera.org/license.txt for license and copyright information.
 
 import warnings
 import weakref
@@ -470,6 +470,13 @@ cdef class ThermoPhase(_SolutionBase):
 
         return index
 
+    property case_sensitive_species_names:
+        """Enforce case-sensitivity for look up of species names"""
+        def __get__(self):
+            return self.thermo.caseSensitiveSpecies()
+        def __set__(self, val):
+            self.thermo.setCaseSensitiveSpecies(bool(val))
+
     def species(self, k=None):
         """
         Return the `Species` object for species *k*, where *k* is either the
@@ -483,7 +490,8 @@ cdef class ThermoPhase(_SolutionBase):
 
         s = Species(init=False)
         if isinstance(k, (str, bytes)):
-            s._assign(self.thermo.species(stringify(k)))
+            kk = self.thermo.speciesIndex(stringify(k))
+            s._assign(self.thermo.species(<int>kk))
         elif isinstance(k, (int, float)):
             s._assign(self.thermo.species(<int>k))
         else:
diff --git a/src/thermo/Phase.cpp b/src/thermo/Phase.cpp
index 880bdfae9..ffe01fe10 100644
--- a/src/thermo/Phase.cpp
+++ b/src/thermo/Phase.cpp
@@ -4,7 +4,7 @@
  */
 
 // This file is part of Cantera. See License.txt in the top-level directory or
-// at http://www.cantera.org/license.txt for license and copyright information.
+// at https://cantera.org/license.txt for license and copyright information.
 
 #include "cantera/thermo/Phase.h"
 #include "cantera/base/utilities.h"
@@ -21,6 +21,7 @@ Phase::Phase() :
     m_kk(0),
     m_ndim(3),
     m_undefinedElementBehavior(UndefElement::add),
+    m_caseSensitiveSpecies(false),
     m_xml(new XML_Node("phase")),
     m_id("<phase>"),
     m_temp(0.001),
@@ -172,20 +173,47 @@ void Phase::getAtoms(size_t k, double* atomArray) const
     }
 }
 
+size_t Phase::findSpeciesLower(const std::string& name) const
+{
+    size_t loc = npos;
+    std::string nLower = toLowerCopy(name);
+    for (const auto& k : m_speciesIndices) {
+        if (toLowerCopy(k.first) == nLower) {
+            if (loc == npos) {
+                loc = k.second;
+            } else {
+                throw CanteraError("Phase::findSpeciesLower",
+                    "Lowercase species name '{}' is not unique", nLower);
+            }
+        }
+    }
+    return loc;
+}
+
 size_t Phase::speciesIndex(const std::string& nameStr) const
 {
-    size_t loc = getValue(m_speciesIndices, toLowerCopy(nameStr), npos);
+    size_t loc = npos;
+    try {
+        return m_speciesIndices.at(nameStr);
+    } catch (std::out_of_range&) {
+        if (!m_caseSensitiveSpecies) {
+            loc = findSpeciesLower(nameStr);
+        }
+    }
     if (loc == npos && nameStr.find(':') != npos) {
         std::string pn;
-        std::string sn = toLowerCopy(parseSpeciesName(nameStr, pn));
+        std::string sn = parseSpeciesName(nameStr, pn);
         if (pn == "" || pn == m_name || pn == m_id) {
-            return getValue(m_speciesIndices, sn, npos);
-        } else {
-            return npos;
+            try {
+                return m_speciesIndices.at(sn);
+            } catch (std::out_of_range&) {
+                if (!m_caseSensitiveSpecies) {
+                    return findSpeciesLower(sn);
+                }
+            }
         }
-    } else {
-        return loc;
     }
+    return loc;
 }
 
 string Phase::speciesName(size_t k) const
@@ -294,10 +322,18 @@ void Phase::setMoleFractionsByName(const compositionMap& xMap)
     vector_fp mf(m_kk, 0.0);
     for (const auto& sp : xMap) {
         try {
-            mf[m_speciesIndices.at(toLowerCopy(sp.first))] = sp.second;
+            mf[m_speciesIndices.at(sp.first)] = sp.second;
         } catch (std::out_of_range&) {
-            throw CanteraError("Phase::setMoleFractionsByName",
-                               "Unknown species '{}'", sp.first);
+            size_t loc = npos;
+            if (!m_caseSensitiveSpecies) {
+                loc = findSpeciesLower(sp.first);
+            }
+            if (loc == npos) {
+                throw CanteraError("Phase::setMoleFractionsByName",
+                                   "Unknown species '{}'", sp.first);
+            } else {
+                mf[loc] = sp.second;
+            }
         }
     }
     setMoleFractions(&mf[0]);
@@ -338,10 +374,18 @@ void Phase::setMassFractionsByName(const compositionMap& yMap)
     vector_fp mf(m_kk, 0.0);
     for (const auto& sp : yMap) {
         try {
-            mf[m_speciesIndices.at(toLowerCopy(sp.first))] = sp.second;
+            mf[m_speciesIndices.at(sp.first)] = sp.second;
         } catch (std::out_of_range&) {
-            throw CanteraError("Phase::setMassFractionsByName",
-                               "Unknown species '{}'", sp.first);
+            size_t loc = npos;
+            if (!m_caseSensitiveSpecies) {
+                loc = findSpeciesLower(sp.first);
+            }
+            if (loc == npos) {
+                throw CanteraError("Phase::setMassFractionsByName",
+                                   "Unknown species '{}'", sp.first);
+            } else {
+                mf[loc] = sp.second;
+            }
         }
     }
     setMassFractions(&mf[0]);
@@ -699,7 +743,8 @@ size_t Phase::addElement(const std::string& symbol, doublereal weight,
 }
 
 bool Phase::addSpecies(shared_ptr<Species> spec) {
-    if (m_species.find(toLowerCopy(spec->name)) != m_species.end()) {
+    // species names are case sensitive
+    if (m_species.find(spec->name) != m_species.end()) {
         throw CanteraError("Phase::addSpecies",
             "Phase '{}' already contains a species named '{}'.",
             m_name, spec->name);
@@ -729,8 +774,8 @@ bool Phase::addSpecies(shared_ptr<Species> spec) {
     }
 
     m_speciesNames.push_back(spec->name);
-    m_species[toLowerCopy(spec->name)] = spec;
-    m_speciesIndices[toLowerCopy(spec->name)] = m_kk;
+    m_species[spec->name] = spec;
+    m_speciesIndices[spec->name] = m_kk;
     m_speciesCharge.push_back(spec->charge);
     size_t ne = nElements();
 
@@ -794,24 +839,28 @@ void Phase::modifySpecies(size_t k, shared_ptr<Species> spec)
             "New species name '{}' does not match existing name '{}'",
                            spec->name, speciesName(k));
     }
-    const shared_ptr<Species>& old = m_species[toLowerCopy(spec->name)];
+    const shared_ptr<Species>& old = m_species[spec->name];
     if (spec->composition != old->composition) {
         throw CanteraError("Phase::modifySpecies",
             "New composition for '{}' does not match existing composition",
             spec->name);
     }
-    m_species[toLowerCopy(spec->name)] = spec;
+    m_species[spec->name] = spec;
     invalidateCache();
 }
 
+//! @deprecated To be removed after Cantera 2.5.
 shared_ptr<Species> Phase::species(const std::string& name) const
 {
-    return m_species.at(toLowerCopy(name));
+    warn_deprecated("Phase::species(std::string&)",
+                    "To be removed after Cantera 2.5. "
+                    "Use Phase::species(size_t) instead.");
+    return m_species.at(name);
 }
 
 shared_ptr<Species> Phase::species(size_t k) const
 {
-    return species(m_speciesNames[k]);
+    return m_species.at(m_speciesNames[k]);
 }
 
 void Phase::ignoreUndefinedElements() {
diff --git a/src/transport/GasTransport.cpp b/src/transport/GasTransport.cpp
index f8447a884..6cc8222bc 100644
--- a/src/transport/GasTransport.cpp
+++ b/src/transport/GasTransport.cpp
@@ -395,7 +395,7 @@ void GasTransport::setupCollisionIntegral()
 void GasTransport::getTransportData()
 {
     for (size_t k = 0; k < m_thermo->nSpecies(); k++) {
-        shared_ptr<Species> s = m_thermo->species(m_thermo->speciesName(k));
+        shared_ptr<Species> s = m_thermo->species(k);
         const GasTransportData* sptran =
             dynamic_cast<GasTransportData*>(s->transport.get());
         if (!sptran) {