diff --git a/epanet/epanet.py b/epanet/epanet.py
index 938780b..77314fa 100644
--- a/epanet/epanet.py
+++ b/epanet/epanet.py
@@ -128,6 +128,7 @@ class Output:
             for j in range(length.value):
                 d |= { category[j] : values[j] }
             usages.append(d)
+            self._lib.ENR_free(ctypes.byref(values))
         return usages
 
 
@@ -140,6 +141,7 @@ class Output:
         d = {}
         for i in range(4):
             d[category[i]] = values[i]
+        self._lib.ENR_free(ctypes.byref(values))
         return d
 
 
@@ -160,6 +162,7 @@ class Output:
                 for k in range(length.value):
                     attributes[category[k]] = values[k]
                 d['result'].append(attributes)
+                self._lib.ENR_free(ctypes.byref(values))
             ds.append(d)
         return ds
 
@@ -188,6 +191,7 @@ class Output:
                         continue
                     attributes[category[k]] = values[k]
                 d['result'].append(attributes)
+                self._lib.ENR_free(ctypes.byref(values))
             ds.append(d)
         return ds