diff --git a/AMDiS/src/FixVec.h b/AMDiS/src/FixVec.h
index 33c6e873da0ab8e85f7f8764a4c532b9faf41cbd..dfa76cae772575e8dd726670a469b74a5e1c9cf0 100644
--- a/AMDiS/src/FixVec.h
+++ b/AMDiS/src/FixVec.h
@@ -440,7 +440,7 @@ namespace AMDiS {
     }
 
     /// Multplication of a matrix with a vector.
-    void multMatrixVec(WorldMatrix<T> &m, WorldVector<T> &v);
+    void multMatrixVec(const WorldMatrix<T> &m, const WorldVector<T> &v);
   };
 
 
diff --git a/AMDiS/src/FixVec.hh b/AMDiS/src/FixVec.hh
index 5990c026eed0944be5c7b8087d7be07e6871c5f7..38407269b5f8a3605f3a3001e081cc1df8ec240f 100644
--- a/AMDiS/src/FixVec.hh
+++ b/AMDiS/src/FixVec.hh
@@ -37,7 +37,7 @@ namespace AMDiS {
   }
 
   template<typename T>
-  void WorldVector<T>::multMatrixVec(WorldMatrix<T> &m, WorldVector<T> &v) 
+  void WorldVector<T>::multMatrixVec(const WorldMatrix<T> &m, const WorldVector<T> &v)
   {
     FUNCNAME("WorldVector<T>::multMatrix()");
     TEST_EXIT_DBG(m.getNumRows() == this->getSize())("invalide size\n");