diff --git a/eval.c b/eval.c
index 15232557..6b7dd202 100644
--- a/eval.c
+++ b/eval.c
@@ -399,6 +399,18 @@ static sexp sexp_make_macro (sexp ctx, sexp p, sexp e) {
   return mac;
 }
 
+sexp sexp_make_variable_transformer_op (sexp ctx, sexp self, sexp_sint_t n, sexp base_proc) {
+  sexp_assert_type(ctx, sexp_procedurep, SEXP_PROCEDURE, base_proc);
+  if (sexp_procedure_variable_transformer_p(base_proc))
+    return base_proc;
+  sexp vt_proc = sexp_alloc_type(ctx, procedure, SEXP_PROCEDURE);
+  sexp_procedure_flags(vt_proc) = (char) (sexp_uint_t) sexp_make_fixnum (sexp_unbox_fixnum(sexp_procedure_flags(base_proc)) | SEXP_PROC_VARIABLE_TRANSFORMER);
+  sexp_procedure_num_args(vt_proc) = sexp_procedure_num_args(base_proc);
+  sexp_procedure_code(vt_proc) =  sexp_procedure_code(base_proc);
+  sexp_procedure_vars(vt_proc) = sexp_procedure_vars(base_proc);
+  return vt_proc;
+}
+
 sexp sexp_make_synclo_op (sexp ctx, sexp self, sexp_sint_t n, sexp env, sexp fv, sexp expr) {
   sexp res;
   sexp_assert_type(ctx, sexp_envp, SEXP_ENV, env);
@@ -766,6 +778,22 @@ static sexp analyze_seq (sexp ctx, sexp ls, int depth, int defok) {
   return res;
 }
 
+static sexp analyze_macro_once (sexp ctx, sexp x, sexp op, int depth) {
+  sexp res;
+  sexp_gc_var1(tmp);
+  sexp_gc_preserve1(ctx, tmp);
+  tmp = sexp_cons(ctx, sexp_macro_env(op), SEXP_NULL);
+  tmp = sexp_cons(ctx, sexp_context_env(ctx), tmp);
+  tmp = sexp_cons(ctx, x, tmp);
+  res = sexp_exceptionp(tmp) ? tmp : sexp_make_child_context(ctx, sexp_context_lambda(ctx));
+  if (!sexp_exceptionp(res) && !sexp_exceptionp(sexp_context_exception(ctx)))
+    res = sexp_apply(res, sexp_macro_proc(op), tmp);
+  if (sexp_exceptionp(res) && sexp_not(sexp_exception_source(x)))
+    sexp_exception_source(res) = sexp_pair_source(sexp_car(tmp));
+  sexp_gc_release1(ctx);
+  return res;
+}
+
 static sexp analyze_var_ref (sexp ctx, sexp x, sexp *varenv) {
   sexp env = sexp_context_env(ctx), res;
   sexp_gc_var1(cell);
@@ -783,31 +811,43 @@ static sexp analyze_var_ref (sexp ctx, sexp x, sexp *varenv) {
   return res;
 }
 
-static sexp analyze_set (sexp ctx, sexp x, int depth) {
+static sexp analyze_set (sexp ctx, sexp x, int depth, int defok) {
   sexp res, varenv;
-  sexp_gc_var2(ref, value);
-  sexp_gc_preserve2(ctx, ref, value);
+  sexp_gc_var4(ref, value, cell, op);
+  sexp_gc_preserve4(ctx, ref, value, cell, op);
   if (! (sexp_pairp(sexp_cdr(x)) && sexp_pairp(sexp_cddr(x))
          && sexp_nullp(sexp_cdddr(x)) && sexp_idp(sexp_cadr(x)))) {
     res = sexp_compile_error(ctx, "bad set! syntax", x);
   } else {
-    ref = analyze_var_ref(ctx, sexp_cadr(x), &varenv);
-    if (sexp_refp(ref) && sexp_lambdap(sexp_ref_loc(ref)))
-      sexp_insert(ctx, sexp_lambda_sv(sexp_ref_loc(ref)), sexp_ref_name(ref));
-    value = analyze(ctx, sexp_caddr(x), depth, 0);
-    if (sexp_exceptionp(ref)) {
-      res = ref;
-    } else if (sexp_exceptionp(value)) {
-      res = value;
-    } else if (sexp_immutablep(sexp_ref_cell(ref))
-               || (varenv && sexp_immutablep(varenv))) {
-      res = sexp_compile_error(ctx, "immutable binding", sexp_cadr(x));
+    cell = sexp_env_cell(ctx, sexp_context_env(ctx), sexp_cadr(x), 0);
+    op = cell ? sexp_cdr(cell) : NULL;
+    if (op && sexp_macrop(op)) {
+      if (!sexp_procedure_variable_transformer_p(sexp_macro_proc(op))) {
+        res = sexp_compile_error(ctx, "can't mutate a syntax keyword", sexp_cadr(x));
+      } else {
+        res = analyze_macro_once(ctx, x, op, depth);
+        if (!sexp_exceptionp(res))
+          res = analyze(ctx, res, depth, defok);
+      }
     } else {
-      res = sexp_make_set(ctx, ref, value);
-      sexp_set_source(res) = sexp_pair_source(x);
+      ref = analyze_var_ref(ctx, sexp_cadr(x), &varenv);
+      if (sexp_refp(ref) && sexp_lambdap(sexp_ref_loc(ref)))
+        sexp_insert(ctx, sexp_lambda_sv(sexp_ref_loc(ref)), sexp_ref_name(ref));
+      value = analyze(ctx, sexp_caddr(x), depth, 0);
+      if (sexp_exceptionp(ref)) {
+        res = ref;
+      } else if (sexp_exceptionp(value)) {
+        res = value;
+      } else if (sexp_immutablep(sexp_ref_cell(ref))
+                 || (varenv && sexp_immutablep(varenv))) {
+        res = sexp_compile_error(ctx, "immutable binding", sexp_cadr(x));
+      } else {
+        res = sexp_make_set(ctx, ref, value);
+        sexp_set_source(res) = sexp_pair_source(x);
+      }
     }
   }
-  sexp_gc_release2(ctx);
+  sexp_gc_release4(ctx);
   return res;
 }
 
@@ -1051,22 +1091,6 @@ static sexp analyze_letrec_syntax (sexp ctx, sexp x, int depth) {
   return analyze_let_syntax_aux(ctx, x, 1, depth);
 }
 
-static sexp analyze_macro_once (sexp ctx, sexp x, sexp op, int depth) {
-  sexp res;
-  sexp_gc_var1(tmp);
-  sexp_gc_preserve1(ctx, tmp);
-  tmp = sexp_cons(ctx, sexp_macro_env(op), SEXP_NULL);
-  tmp = sexp_cons(ctx, sexp_context_env(ctx), tmp);
-  tmp = sexp_cons(ctx, x, tmp);
-  res = sexp_exceptionp(tmp) ? tmp : sexp_make_child_context(ctx, sexp_context_lambda(ctx));
-  if (!sexp_exceptionp(res) && !sexp_exceptionp(sexp_context_exception(ctx)))
-    res = sexp_apply(res, sexp_macro_proc(op), tmp);
-  if (sexp_exceptionp(res) && sexp_not(sexp_exception_source(x)))
-    sexp_exception_source(res) = sexp_pair_source(sexp_car(tmp));
-  sexp_gc_release1(ctx);
-  return res;
-}
-
 static sexp analyze (sexp ctx, sexp object, int depth, int defok) {
   sexp op;
   sexp_gc_var4(res, tmp, x, cell);
@@ -1100,7 +1124,7 @@ static sexp analyze (sexp ctx, sexp object, int depth, int defok) {
               : sexp_compile_error(ctx, "unexpected define", x);
             break;
           case SEXP_CORE_SET:
-            res = analyze_set(ctx, x, depth); break;
+            res = analyze_set(ctx, x, depth, defok); break;
           case SEXP_CORE_LAMBDA:
             res = analyze_lambda(ctx, x, depth); break;
           case SEXP_CORE_IF:
diff --git a/include/chibi/eval.h b/include/chibi/eval.h
index ebbad05d..bbd905cf 100644
--- a/include/chibi/eval.h
+++ b/include/chibi/eval.h
@@ -128,6 +128,7 @@ SEXP_API sexp sexp_warn_undefs_op (sexp ctx, sexp self, sexp_sint_t n, sexp from
 SEXP_API sexp sexp_make_lit (sexp ctx, sexp value);
 SEXP_API sexp sexp_make_opcode (sexp, sexp, sexp, sexp, sexp, sexp, sexp, sexp, sexp, sexp, sexp, sexp, sexp_proc1);
 SEXP_API sexp sexp_make_procedure_op (sexp ctx, sexp self, sexp_sint_t n, sexp flags, sexp num_args, sexp bc, sexp vars);
+SEXP_API sexp sexp_make_variable_transformer_op (sexp ctx, sexp self, sexp_sint_t n, sexp base_proc);
 SEXP_API sexp sexp_define_foreign_aux (sexp ctx, sexp env, const char *name, int num_args, int flags, const char *fname, sexp_proc1 f, sexp data);
 SEXP_API sexp sexp_register_optimization(sexp ctx, sexp self, sexp_sint_t n, sexp f, sexp i);
 #if SEXP_USE_AUTO_FORCE
diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index 86435616..3a06fce5 100644
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -270,6 +270,7 @@ typedef int sexp_sint_t;
 #define SEXP_PROC_NONE ((sexp_uint_t)0)
 #define SEXP_PROC_VARIADIC ((sexp_uint_t)1)
 #define SEXP_PROC_UNUSED_REST ((sexp_uint_t)2)
+#define SEXP_PROC_VARIABLE_TRANSFORMER ((sexp_uint_t)4)
 
 
 #ifdef SEXP_USE_INTTYPES
@@ -1146,6 +1147,7 @@ SEXP_API unsigned long long sexp_bignum_to_uint(sexp x);
 #define sexp_procedure_flags(x)      (sexp_field(x, procedure, SEXP_PROCEDURE, flags))
 #define sexp_procedure_variadic_p(x) (sexp_unbox_fixnum(sexp_procedure_flags(x)) & SEXP_PROC_VARIADIC)
 #define sexp_procedure_unused_rest_p(x) (sexp_unbox_fixnum(sexp_procedure_flags(x)) & SEXP_PROC_UNUSED_REST)
+#define sexp_procedure_variable_transformer_p(x) (sexp_unbox_fixnum(sexp_procedure_flags(x)) & SEXP_PROC_VARIABLE_TRANSFORMER)
 #define sexp_procedure_code(x)       (sexp_field(x, procedure, SEXP_PROCEDURE, bc))
 #define sexp_procedure_vars(x)       (sexp_field(x, procedure, SEXP_PROCEDURE, vars))
 #define sexp_procedure_source(x)     sexp_bytecode_source(sexp_procedure_code(x))
diff --git a/opcodes.c b/opcodes.c
index 552fc698..fc66db3e 100644
--- a/opcodes.c
+++ b/opcodes.c
@@ -197,6 +197,7 @@ _FN2(_I(SEXP_OBJECT), _I(SEXP_OBJECT), SEXP_NULL, "memq", 0, sexp_memq_op),
 _FN2(_I(SEXP_OBJECT), _I(SEXP_OBJECT), SEXP_NULL, "assq", 0, sexp_assq_op),
 _FN3(_I(SEXP_SYNCLO), _I(SEXP_ENV), SEXP_NULL, _I(SEXP_OBJECT), "make-syntactic-closure", 0, sexp_make_synclo_op),
 _FN1(_I(SEXP_OBJECT), _I(SEXP_OBJECT), "strip-syntactic-closures", 0, sexp_strip_synclos),
+_FN1(_I(SEXP_PROCEDURE), _I(SEXP_PROCEDURE), "make-variable-transformer", 0, sexp_make_variable_transformer_op),
 _FN0(_I(SEXP_OPORT), "open-output-string", 0, sexp_open_output_string_op),
 _FN1(_I(SEXP_IPORT), _I(SEXP_STRING), "open-input-string", 0, sexp_open_input_string_op),
 _FN1(_I(SEXP_STRING), _I(SEXP_OPORT), "get-output-string", 0, sexp_get_output_string_op),
diff --git a/sexp.c b/sexp.c
index 2fbdb2f1..4da3b01a 100644
--- a/sexp.c
+++ b/sexp.c
@@ -2190,6 +2190,8 @@ sexp sexp_write_one (sexp ctx, sexp obj, sexp out, sexp_sint_t bound) {
 #endif
     case SEXP_PROCEDURE:
       sexp_write_string(ctx, "#<procedure ", out);
+      if (sexp_procedure_variable_transformer_p(obj))
+        sexp_write_string(ctx, "(variable-transformer) ", out);
       x = sexp_bytecode_name(sexp_procedure_code(obj));
       sexp_write_one(ctx, sexp_synclop(x) ? sexp_synclo_expr(x): x, out, bound+1);
       sexp_write_string(ctx, " ", out);