diff --git a/src/mat4.c b/src/mat4.c
index 8fcc9c24..5121140f 100644
--- a/src/mat4.c
+++ b/src/mat4.c
@@ -110,7 +110,7 @@ Mat_Create4(const char *matname)
  * @endif
  */
 int
-Mat_VarWrite4(mat_t *mat, const matvar_t *matvar)
+Mat_VarWrite4(const mat_t *mat, const matvar_t *matvar)
 {
 #if defined(MATIO_LE) || defined(MATIO_BE)
     typedef struct
diff --git a/src/mat4.h b/src/mat4.h
index 1e81b41a..801c9bb4 100644
--- a/src/mat4.h
+++ b/src/mat4.h
@@ -36,7 +36,7 @@
 
 EXTERN mat_t *Mat_Create4(const char *matname);
 
-EXTERN int Mat_VarWrite4(mat_t *mat, const matvar_t *matvar);
+EXTERN int Mat_VarWrite4(const mat_t *mat, const matvar_t *matvar);
 EXTERN int Mat_VarRead4(mat_t *mat, matvar_t *matvar);
 EXTERN int Mat_VarReadData4(mat_t *mat, const matvar_t *matvar, void *data, const int *start,
                             const int *stride, const int *edge);