diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
index 4f450da..ff278cf 100644
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -417,7 +417,8 @@
 static PyObject *
 err_mode(char *action)
 {
-    PyErr_Format(PyExc_ValueError, "File not open for %s", action);
+    PyErr_Format(IO_STATE->unsupported_operation,
+                 "File not open for %s", action);
     return NULL;
 }
 
diff --git a/Modules/_io/iobase.c b/Modules/_io/iobase.c
index 2d664ab..6521ae2 100644
--- a/Modules/_io/iobase.c
+++ b/Modules/_io/iobase.c
@@ -317,7 +317,7 @@
         return NULL;
     if (res != Py_True) {
         Py_CLEAR(res);
-        PyErr_SetString(PyExc_IOError, "File or stream is not seekable.");
+        iobase_unsupported("File or stream is not seekable.");
         return NULL;
     }
     if (args == Py_True) {
@@ -346,7 +346,7 @@
         return NULL;
     if (res != Py_True) {
         Py_CLEAR(res);
-        PyErr_SetString(PyExc_IOError, "File or stream is not readable.");
+        iobase_unsupported("File or stream is not readable.");
         return NULL;
     }
     if (args == Py_True) {
@@ -375,7 +375,7 @@
         return NULL;
     if (res != Py_True) {
         Py_CLEAR(res);
-        PyErr_SetString(PyExc_IOError, "File or stream is not writable.");
+        iobase_unsupported("File or stream is not writable.");
         return NULL;
     }
     if (args == Py_True) {
diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index b659795..08827b9 100644
--- a/Modules/_io/textio.c
+++ b/Modules/_io/textio.c
@@ -1259,10 +1259,8 @@
 
     CHECK_CLOSED(self);
 
-    if (self->encoder == NULL) {
-        PyErr_SetString(PyExc_IOError, "not writable");
-        return NULL;
-    }
+    if (self->encoder == NULL)
+        return _unsupported("not writable");
 
     Py_INCREF(text);
 
@@ -1399,7 +1397,7 @@
      */
 
     if (self->decoder == NULL) {
-        PyErr_SetString(PyExc_IOError, "not readable");
+        _unsupported("not readable");
         return -1;
     }
 
@@ -1489,10 +1487,8 @@
 
     CHECK_CLOSED(self);
 
-    if (self->decoder == NULL) {
-        PyErr_SetString(PyExc_IOError, "not readable");
-        return NULL;
-    }
+    if (self->decoder == NULL)
+        return _unsupported("not readable");
 
     if (_textiowrapper_writeflush(self) < 0)
         return NULL;
@@ -1983,8 +1979,7 @@
     Py_INCREF(cookieObj);
 
     if (!self->seekable) {
-        PyErr_SetString(PyExc_IOError,
-                        "underlying stream is not seekable");
+        _unsupported("underlying stream is not seekable");
         goto fail;
     }
 
@@ -1995,8 +1990,7 @@
             goto fail;
 
         if (cmp == 0) {
-            PyErr_SetString(PyExc_IOError,
-                            "can't do nonzero cur-relative seeks");
+            _unsupported("can't do nonzero cur-relative seeks");
             goto fail;
         }
 
@@ -2016,8 +2010,7 @@
             goto fail;
 
         if (cmp == 0) {
-            PyErr_SetString(PyExc_IOError,
-                            "can't do nonzero end-relative seeks");
+            _unsupported("can't do nonzero end-relative seeks");
             goto fail;
         }
 
@@ -2151,8 +2144,7 @@
     CHECK_CLOSED(self);
 
     if (!self->seekable) {
-        PyErr_SetString(PyExc_IOError,
-                        "underlying stream is not seekable");
+        _unsupported("underlying stream is not seekable");
         goto fail;
     }
     if (!self->telling) {