diff --git a/thermosteam/_phase.py b/thermosteam/_phase.py
index 9873d76a..eed9de35 100644
--- a/thermosteam/_phase.py
+++ b/thermosteam/_phase.py
@@ -110,7 +110,14 @@ def __call__(self, phase):
         try:
             return self._index[phase]
         except:
-            raise UndefinedPhase(phase)        
+            if phase.isupper():
+                phase = phase.lower()
+            else:
+                phase = phase.upper()
+            try:
+                return self._index[phase]
+            except:
+                raise UndefinedPhase(phase)        
     
     @property
     def phases(self):