diff --git a/src/templates_front/templator_front.org b/src/templates_front/templator_front.org
index f613a49..8a217b8 100644
--- a/src/templates_front/templator_front.org
+++ b/src/templates_front/templator_front.org
@@ -451,7 +451,6 @@ struct trexio_back_end_s {
      2) ~mode~          - character containing open mode (see below)
        - ~'w'~         - (write)  creates a new file as READWRITE (overwrite existing file)
        - ~'r'~         - (read)   opens existing file as READONLY
-       - ~'a'~         - (append) either opens file in READWRITE mode if it already exists or creates a new one
      3) ~back_end~      - integer number (or the corresponding global parameter) specifying the back end
        - ~TREXIO_HDF5~ - for HDF5 back end (integer alternative: 0)
        - ~TREXIO_TEXT~ - for TEXT back end (integer alternative: 1)
@@ -482,7 +481,7 @@ trexio_open(const char* file_name, const char mode,
   if (back_end <  0) return NULL;
   if (back_end >= TREXIO_INVALID_BACK_END) return NULL;
 
-  if (mode != 'r' && mode != 'w' && mode != 'a') return NULL;
+  if (mode != 'r' && mode != 'w') return NULL;
 
   trexio_t* result = NULL;
   void* result_tmp = NULL;
diff --git a/src/templates_hdf5/templator_hdf5.org b/src/templates_hdf5/templator_hdf5.org
index 1954539..cb5faa9 100644
--- a/src/templates_hdf5/templator_hdf5.org
+++ b/src/templates_hdf5/templator_hdf5.org
@@ -97,10 +97,6 @@ trexio_hdf5_init (trexio_t* const file)
       // reading the existing file -> open as RDONLY
       f->file_id = H5Fopen(file->file_name, H5F_ACC_RDONLY, H5P_DEFAULT);
       break;
-    case 'a':
-      // appending the existing file -> open as RDWR
-      f->file_id = H5Fopen(file->file_name, H5F_ACC_RDWR, H5P_DEFAULT);
-      break;
     case 'w':
       // writing the existing file -> overwrite it (_TRUNC) [_EXCL | H5F_ACC_DEBUG as an alternative]
       f->file_id = H5Fcreate(file->file_name, H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT);
@@ -111,8 +107,7 @@ trexio_hdf5_init (trexio_t* const file)
 
     switch (file->mode) {
     case 'r':
-    case 'a':
-      // reading or appending non-existing file -> error
+      // reading non-existing file -> error
       return TREXIO_FAILURE;
     case 'w':
       // writing non-existing file -> create it
@@ -124,9 +119,7 @@ trexio_hdf5_init (trexio_t* const file)
 
   /* Create or open groups in the hdf5 file assuming that they exist if file exists */
   switch (file->mode) {
-    // the switch for 'r'/'a' is reached only if file exists
     case 'r':
-    case 'a':
       f->$group$_group = H5Gopen(f->file_id, $GROUP$_GROUP_NAME, H5P_DEFAULT);
       break;
     case 'w':