diff --git a/eval.c b/eval.c
index 871feb9d..7e599d5d 100644
--- a/eval.c
+++ b/eval.c
@@ -331,6 +331,7 @@ static sexp sexp_strip_synclos (sexp ctx, sexp x) {
     kar = sexp_strip_synclos(ctx, sexp_car(x));
     kdr = sexp_strip_synclos(ctx, sexp_cdr(x));
     res = sexp_cons(ctx, kar, kdr);
+    sexp_immutablep(res) = sexp_immutablep(x);
   } else {
     res = x;
   }
@@ -1497,6 +1498,8 @@ sexp sexp_vm (sexp ctx, sexp proc) {
   case OP_VECTOR_SET:
     if (! sexp_vectorp(_ARG1))
       sexp_raise("vector-set!: not a vector", sexp_list1(ctx, _ARG1));
+    else if (sexp_immutablep(_ARG1))
+      sexp_raise("vector-set!: immutable vector", sexp_list1(ctx, _ARG1));
     sexp_vector_set(_ARG1, _ARG2, _ARG3);
     _ARG3 = SEXP_VOID;
     top-=2;
@@ -1509,6 +1512,11 @@ sexp sexp_vm (sexp ctx, sexp proc) {
     top--;
     break;
   case OP_STRING_SET:
+    if (! sexp_stringp(_ARG1))
+      sexp_raise("string-set!: not a string", sexp_list1(ctx, _ARG1));
+    else if (sexp_immutablep(_ARG1))
+      sexp_raise("string-set!: immutable string", sexp_list1(ctx, _ARG1));
+    fprintf(stderr, "string-set! %p (immutable: %d)\n", _ARG1, sexp_immutablep(_ARG1));
     sexp_string_set(_ARG1, _ARG2, _ARG3);
     _ARG3 = SEXP_VOID;
     top-=2;
@@ -1557,6 +1565,8 @@ sexp sexp_vm (sexp ctx, sexp proc) {
   case OP_SET_CAR:
     if (! sexp_pairp(_ARG1))
       sexp_raise("set-car!: not a pair", sexp_list1(ctx, _ARG1));
+    else if (sexp_immutablep(_ARG1))
+      sexp_raise("set-car!: immutable pair", sexp_list1(ctx, _ARG1));
     sexp_car(_ARG1) = _ARG2;
     _ARG2 = SEXP_VOID;
     top--;
@@ -1564,6 +1574,8 @@ sexp sexp_vm (sexp ctx, sexp proc) {
   case OP_SET_CDR:
     if (! sexp_pairp(_ARG1))
       sexp_raise("set-cdr!: not a pair", sexp_list1(ctx, _ARG1));
+    else if (sexp_immutablep(_ARG1))
+      sexp_raise("set-cdr!: immutable pair", sexp_list1(ctx, _ARG1));
     sexp_cdr(_ARG1) = _ARG2;
     _ARG2 = SEXP_VOID;
     top--;
diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index 8ab96d98..d1538cf2 100644
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -286,6 +286,7 @@ void *sexp_realloc(sexp ctx, sexp x, size_t size);
 
 #define sexp_pointer_tag(x) ((x)->tag)
 #define sexp_gc_mark(x)     ((x)->gc_mark)
+#define sexp_immutablep(x)  ((x)->immutablep)
 
 #define sexp_check_tag(x,t) (sexp_pointerp(x) && (sexp_pointer_tag(x) == (t)))
 
diff --git a/sexp.c b/sexp.c
index 9b909936..7d1d12d9 100644
--- a/sexp.c
+++ b/sexp.c
@@ -894,9 +894,10 @@ void sexp_write (sexp ctx, sexp obj, sexp out) {
     else if (obj == sexp_make_character('\t'))
       sexp_write_string(ctx, "#\\tab", out);
     else if ((33 <= sexp_unbox_character(obj))
-             && (sexp_unbox_character(obj) < 127))
+             && (sexp_unbox_character(obj) < 127)) {
+      sexp_write_string(ctx, "#\\", out);
       sexp_write_char(ctx, sexp_unbox_character(obj), out);
-    else {
+    } else {
       sexp_write_string(ctx, "#\\x", out);
       sexp_write_char(ctx, hex_digit(sexp_unbox_character(obj)>>4), out);
       sexp_write_char(ctx, hex_digit(sexp_unbox_character(obj)&0xF), out);
@@ -1156,9 +1157,13 @@ sexp sexp_read_raw (sexp ctx, sexp in) {
         res = sexp_read_error(ctx, "missing trailing ')'", SEXP_NULL, in);
       }
     }
-    if ((line >= 0) && sexp_pairp(res))
+    if ((line >= 0) && sexp_pairp(res)) {
       sexp_pair_source(res)
         = sexp_cons(ctx, sexp_port_name(in), sexp_make_integer(line));
+    }
+    if (sexp_port_sourcep(in))
+      for (tmp=res; sexp_pairp(tmp); tmp=sexp_cdr(tmp))
+        sexp_immutablep(tmp) = 1;
     break;
   case '#':
     switch (c1=sexp_read_char(ctx, in)) {
@@ -1296,6 +1301,8 @@ sexp sexp_read_raw (sexp ctx, sexp in) {
     break;
   }
 
+  if (sexp_port_sourcep(in) && sexp_pointerp(res))
+    sexp_immutablep(res) = 1;
   sexp_gc_release(ctx, res, s_res);
   return res;
 }