diff --git a/libguile/ports.c b/libguile/ports.c
index f5d528462..8799acaa6 100644
--- a/libguile/ports.c
+++ b/libguile/ports.c
@@ -2580,6 +2580,11 @@ scm_i_set_port_encoding_x (SCM port, const char *encoding)
      because we do I/O ourselves.  This saves 100+ KiB for each
      descriptor.  */
   pt->encoding = scm_gc_strdup (encoding, "port");
+
+  /* IMPORTANT: If the set of encoding modes is changed, or if more
+     would need to be done after setting pt->encoding = NULL, then
+     update 'make_bip', 'make_cbip', 'make_bop', and 'make_cbop' in
+     r6rs-ports.c accordingly.  This will be cleaned up in 2.2.  */
   if (c_strcasecmp (encoding, "UTF-8") == 0)
     pti->encoding_mode = SCM_PORT_ENCODING_MODE_UTF8;
   else
diff --git a/libguile/r6rs-ports.c b/libguile/r6rs-ports.c
index 93171f06d..a17b7b408 100644
--- a/libguile/r6rs-ports.c
+++ b/libguile/r6rs-ports.c
@@ -89,6 +89,8 @@ make_bip (SCM bv)
 
   /* Match the expectation of `binary-port?'.  */
   c_port->encoding = NULL;
+  /* XXX Manually update encoding_mode.  This will be cleaned up in 2.2.  */
+  SCM_PORT_GET_INTERNAL (port)->encoding_mode = SCM_PORT_ENCODING_MODE_ICONV;
 
   /* Prevent BV from being GC'd.  */
   SCM_SETSTREAM (port, SCM_UNPACK (bv));
@@ -362,6 +364,8 @@ make_cbip (SCM read_proc, SCM get_position_proc,
 
   /* Match the expectation of `binary-port?'.  */
   c_port->encoding = NULL;
+  /* XXX Manually update encoding_mode.  This will be cleaned up in 2.2.  */
+  SCM_PORT_GET_INTERNAL (port)->encoding_mode = SCM_PORT_ENCODING_MODE_ICONV;
 
   /* Attach it the method vector.  */
   SCM_SETSTREAM (port, SCM_UNPACK (method_vector));
@@ -912,6 +916,8 @@ make_bop (void)
 
   /* Match the expectation of `binary-port?'.  */
   c_port->encoding = NULL;
+  /* XXX Manually update encoding_mode.  This will be cleaned up in 2.2.  */
+  SCM_PORT_GET_INTERNAL (port)->encoding_mode = SCM_PORT_ENCODING_MODE_ICONV;
 
   buf = (scm_t_bop_buffer *) scm_gc_malloc (sizeof (* buf), SCM_GC_BOP);
   bop_buffer_init (buf);
@@ -1071,6 +1077,8 @@ make_cbop (SCM write_proc, SCM get_position_proc,
 
   /* Match the expectation of `binary-port?'.  */
   c_port->encoding = NULL;
+  /* XXX Manually update encoding_mode.  This will be cleaned up in 2.2.  */
+  SCM_PORT_GET_INTERNAL (port)->encoding_mode = SCM_PORT_ENCODING_MODE_ICONV;
 
   /* Attach it the method vector.  */
   SCM_SETSTREAM (port, SCM_UNPACK (method_vector));
diff --git a/test-suite/tests/r6rs-ports.test b/test-suite/tests/r6rs-ports.test
index e5f1266a0..7bf9ffa49 100644
--- a/test-suite/tests/r6rs-ports.test
+++ b/test-suite/tests/r6rs-ports.test
@@ -357,6 +357,11 @@
     (with-fluids ((%default-port-encoding "UTF-8"))
       (binary-port? (open-bytevector-input-port #vu8(1 2 3)))))
 
+  (pass-if-equal "bytevector-input-port uses ISO-8859-1 (Guile extension)"
+      "©©"
+    (with-fluids ((%default-port-encoding "UTF-8"))
+      (get-string-all (open-bytevector-input-port #vu8(194 169 194 169)))))
+
   (pass-if-exception "bytevector-input-port is read-only"
     exception:wrong-type-arg
 
@@ -417,6 +422,23 @@
            (input-port? port)
            (bytevector=? (get-bytevector-all port) source))))
 
+  (pass-if-equal "make-custom-binary-input-port uses ISO-8859-1 (Guile extension)"
+      "©©"
+    (with-fluids ((%default-port-encoding "UTF-8"))
+      (let* ((source #vu8(194 169 194 169))
+             (read! (let ((pos 0)
+                          (len (bytevector-length source)))
+                      (lambda (bv start count)
+                        (let ((amount (min count (- len pos))))
+                          (if (> amount 0)
+                              (bytevector-copy! source pos
+                                                bv start amount))
+                          (set! pos (+ pos amount))
+                          amount))))
+             (port (make-custom-binary-input-port "the port" read!
+                                                  #f #f #f)))
+        (get-string-all port))))
+
   (pass-if "custom binary input port does not support `port-position'"
     (let* ((str "Hello Port!")
            (source (open-bytevector-input-port
@@ -717,6 +739,14 @@ not `set-port-position!'"
   (pass-if "bytevector-output-port is binary"
     (binary-port? (open-bytevector-output-port)))
 
+  (pass-if-equal "bytevector-output-port uses ISO-8859-1 (Guile extension)"
+      #vu8(194 169 194 169)
+    (with-fluids ((%default-port-encoding "UTF-8"))
+      (let-values (((port get-content)
+                    (open-bytevector-output-port)))
+        (put-string port "©©")
+        (get-content))))
+
   (pass-if "open-bytevector-output-port [extract after close]"
     (let-values (((port get-content)
                   (open-bytevector-output-port)))
@@ -818,6 +848,22 @@ not `set-port-position!'"
            (not eof?)
            (bytevector=? sink source))))
 
+  (pass-if-equal "custom-binary-output-port uses ISO-8859-1 (Guile extension)"
+      '(194 169 194 169)
+    (with-fluids ((%default-port-encoding "UTF-8"))
+      (let* ((sink '())
+             (write! (lambda (bv start count)
+                       (if (= 0 count)  ; EOF
+                           0
+                           (let ((u8 (bytevector-u8-ref bv start)))
+                             ;; Get one byte at a time.
+                             (set! sink (cons u8 sink))
+                             1))))
+             (port (make-custom-binary-output-port "cbop" write!
+                                                   #f #f #f)))
+      (put-string port "©©")
+      (reverse sink))))
+
   (pass-if "standard-output-port is binary"
     (with-fluids ((%default-port-encoding "UTF-8"))
       (binary-port? (standard-output-port))))