diff --git a/include/cantera/equil/vcs_defs.h b/include/cantera/equil/vcs_defs.h
index 74c00e148..433b30867 100644
--- a/include/cantera/equil/vcs_defs.h
+++ b/include/cantera/equil/vcs_defs.h
@@ -181,11 +181,10 @@ namespace VCSnonideal
  */
 #define VCS_SPECIES_DELETED       -4
 
-//! Species refers to an electron in the metal
+//! Species refers to an electron in the metal. 
 /*!
- *  The unknown is equal to the interfacial voltage
- *  drop across the interface on the SHE (standard
- *  hydrogen electrode) scale (volts).
+ *  The unknown is equal to the electric potential of the phase
+ *  in which it exists.
  */
 #define VCS_SPECIES_INTERFACIALVOLTAGE  -5
 
@@ -363,7 +362,8 @@ namespace VCSnonideal
 /*!
  * Typically, these species are electrons in metals. There is an
  * infinite supply of them. However, their electrical potential
- * is defined by the interface voltage.
+ * is sometimes allowed to vary, for example if the open circuit voltage
+ * is sought after. 
  */
 #define VCS_SPECIES_TYPE_INTERFACIALVOLTAGE -5
 //@}
diff --git a/src/equil/vcs_MultiPhaseEquil.cpp b/src/equil/vcs_MultiPhaseEquil.cpp
index 7cbe3018d..9ade0aece 100644
--- a/src/equil/vcs_MultiPhaseEquil.cpp
+++ b/src/equil/vcs_MultiPhaseEquil.cpp
@@ -1099,24 +1099,32 @@ int  vcs_Cantera_to_vprob(Cantera::MultiPhase* mphase,
              */
             string stmp = mphase->speciesName(kT);
             vprob->SpName[kT] = stmp;
-
             /*
-             *   Set the initial estimate of the number of kmoles of the species
-             *   and the mole fraction vector. translate from
-             *   kmol to gmol.
+             * Transfer the type of unknown
              */
-            vprob->w[kT] = mphase->speciesMoles(kT);
-            tMoles += vprob->w[kT];
-            vprob->mf[kT] = mphase->moleFraction(kT);
+            vprob->SpeciesUnknownType[kT] = VolPhase->speciesUnknownType(k);
+
+            if (vprob->SpeciesUnknownType[kT] == VCS_SPECIES_TYPE_MOLNUM) {
+                /*
+                 *   Set the initial number of kmoles of the species
+                 *   and the mole fraction vector
+                 */
+                vprob->w[kT] = mphase->speciesMoles(kT);
+                tMoles += vprob->w[kT];
+                vprob->mf[kT] = mphase->moleFraction(kT);
+             } else if (vprob->SpeciesUnknownType[kT] == VCS_SPECIES_TYPE_INTERFACIALVOLTAGE) {
+                vprob->w[kT] = tPhase->electricPotential();
+                vprob->mf[kT] = mphase->moleFraction(kT);
+             } else {
+               throw CanteraError(" vcs_Cantera_to_vprob() ERROR",
+                                  "Unknown species type: " + int2str(vprob->SpeciesUnknownType[kT]));
+             }
+
 
             /*
              * transfer chemical potential vector
              */
             vprob->m_gibbsSpecies[kT] = muPhase[k];
-            /*
-             * Transfer the type of unknown
-             */
-            vprob->SpeciesUnknownType[kT] = VolPhase->speciesUnknownType(k);
             /*
              * Transfer the species information from the
              * volPhase structure to the VPROB structure