diff --git a/src/base/ctexceptions.cpp b/src/base/ctexceptions.cpp
index c86060f47..82d958fbf 100644
--- a/src/base/ctexceptions.cpp
+++ b/src/base/ctexceptions.cpp
@@ -54,17 +54,14 @@ std::string CanteraError::getMessage() const
 
 std::string ArraySizeError::getMessage() const
 {
-    std::stringstream ss;
-    ss << "Array size (" << sz_ << ") too small. Must be at least " << reqd_ << ".";
-    return ss.str();
+    return fmt::format("Array size ({}) too small. Must be at least {}.",
+                       sz_, reqd_);
 }
 
 std::string IndexError::getMessage() const
 {
-    std::stringstream ss;
-    ss << "IndexError: " << arrayName_ << "[" << m_ << "]" <<
-       " outside valid range of 0 to " << (mmax_) << ".";
-    return ss.str();
+    return fmt::format("IndexError: {}[{}] outside valid range of 0 to {}.",
+                       arrayName_, m_, mmax_);
 }
 
 } // namespace Cantera
diff --git a/src/equil/vcs_VolPhase.cpp b/src/equil/vcs_VolPhase.cpp
index 0006d9f54..2d0e9d96f 100644
--- a/src/equil/vcs_VolPhase.cpp
+++ b/src/equil/vcs_VolPhase.cpp
@@ -14,7 +14,6 @@
 #include "cantera/thermo/mix_defs.h"
 #include "cantera/base/stringUtils.h"
 
-#include <sstream>
 #include <cstdio>
 
 namespace Cantera
@@ -204,9 +203,7 @@ void vcs_VolPhase::resize(const size_t phaseNum, const size_t nspecies,
     } else {
         VP_ID_ = phaseNum;
         if (!phaseName) {
-            std::stringstream sstmp;
-            sstmp << "Phase_" << VP_ID_;
-            PhaseName = sstmp.str();
+            PhaseName = fmt::format("Phase_{}", VP_ID_);
         } else {
             PhaseName = phaseName;
         }
@@ -1068,9 +1065,7 @@ size_t vcs_VolPhase::transferElementsFM(const ThermoPhase* const tPhase)
     if (cne) {
         std::string pname = tPhase->id();
         if (pname == "") {
-            std::stringstream sss;
-            sss << "phase" << VP_ID_;
-            pname = sss.str();
+            pname = fmt::format("phase{}", VP_ID_);
         }
         e = ChargeNeutralityElement;
         m_elementNames[e] = "cn_" + pname;
diff --git a/src/numerics/CVodesIntegrator.cpp b/src/numerics/CVodesIntegrator.cpp
index 056dd47d1..192dc5835 100644
--- a/src/numerics/CVodesIntegrator.cpp
+++ b/src/numerics/CVodesIntegrator.cpp
@@ -30,8 +30,6 @@ typedef int sd_size_t;
 typedef long int sd_size_t;
 #endif
 
-#include <sstream>
-
 namespace Cantera
 {
 
@@ -460,10 +458,10 @@ string CVodesIntegrator::getErrorInfo(int N)
 
     N = std::min(N, static_cast<int>(m_neq));
     sort(weightedErrors.begin(), weightedErrors.end());
-    stringstream s;
+    fmt::MemoryWriter s;
     for (int i=0; i<N; i++) {
-        s << get<2>(weightedErrors[i]) << ": "
-          << get<1>(weightedErrors[i]) << endl;
+        s.write("{}: {}\n",
+                get<2>(weightedErrors[i]), get<1>(weightedErrors[i]));
     }
     return s.str();
 }