diff --git a/loopy/preprocess.py b/loopy/preprocess.py
index 027e98ae51c7992f98c7ab22f6574259bbca00c8..f93ea891fd7f1aa420c8c109c31b50fd8305a9e3 100644
--- a/loopy/preprocess.py
+++ b/loopy/preprocess.py
@@ -72,6 +72,24 @@ def prepare_for_caching(kernel):
 # }}}
 
 
+# {{{ check for writes to predicates
+
+def check_for_writes_to_predicates(kernel):
+    from loopy.symbolic import get_dependencies
+    for insn in kernel.instructions:
+        pred_vars = (
+                frozenset.union(
+                    *(get_dependencies(pred) for pred in insn.predicates))
+                if insn.predicates else frozenset())
+        written_pred_vars = frozenset(insn.assignee_var_names()) & pred_vars
+        if written_pred_vars:
+            raise LoopyError("In instruction '%s': may not write to "
+                    "variable(s) '%s' involved in the instruction's predicates"
+                    % (insn.id, ", ".join(written_pred_vars)))
+
+# }}}
+
+
 # {{{ check reduction iname uniqueness
 
 def check_reduction_iname_uniqueness(kernel):
@@ -876,6 +894,7 @@ def preprocess_kernel(kernel, device=None):
 
     kernel = infer_unknown_types(kernel, expect_completion=False)
 
+    check_for_writes_to_predicates(kernel)
     check_reduction_iname_uniqueness(kernel)
 
     from loopy.kernel.creation import apply_single_writer_depencency_heuristic