diff --git a/dialects/clos/clos.cpp b/dialects/clos/clos.cpp
index f72f734cb7..a64789aee7 100644
--- a/dialects/clos/clos.cpp
+++ b/dialects/clos/clos.cpp
@@ -20,6 +20,23 @@
 
 namespace thorin::clos {
 
+size_t env_idx(Defs defs) {
+    if (defs.empty()) return 0;
+    if (match<mem::M>(defs.front()) || match<mem::M>(defs.front()->type())) return 1;
+    return 0;
+}
+
+Sigma* doms2clos(World& world, Defs doms) {
+    auto sigma   = world.nom_sigma(world.type<1>(), 3, world.dbg("Clos"))->set(0, world.type());
+    auto env_t   = sigma->var(0_s);
+    auto new_dom = env_insert<true>(world, doms, env_t);
+    sigma->set(1, world.cn(new_dom));
+    sigma->set(2, env_t);
+    return sigma;
+}
+
+Sigma* pi2clos(World& world, const Pi* pi) { return doms2clos(world, pi->doms()); }
+
 /*
  * ClosLit
  */
@@ -99,7 +116,7 @@ const Sigma* isa_clos_type(const Def* def) {
     return (pi && pi->is_cn() && pi->num_ops() > 1_u64 && pi->dom(Clos_Env_Param) == var) ? sig : nullptr;
 }
 
-Sigma* clos_type(const Pi* pi) { return ctype(pi->world(), pi->doms(), nullptr)->as_nom<Sigma>(); }
+Sigma* clos_type(const Pi* pi) { return pi2clos(pi->world(), pi); }
 
 const Pi* clos_type_to_pi(const Def* ct, const Def* new_env_type) {
     assert(isa_clos_type(ct));
diff --git a/dialects/clos/clos.h b/dialects/clos/clos.h
index 7cb6047f55..40d3e00026 100644
--- a/dialects/clos/clos.h
+++ b/dialects/clos/clos.h
@@ -48,9 +48,7 @@ class ClosLit {
     ///@}
 
     operator bool() const { return def_ != nullptr; }
-
     operator const Tuple*() { return def_; }
-
     const Tuple* operator->() {
         assert(def_);
         return def_;
@@ -163,6 +161,22 @@ inline const Def* clos_remove_env(const Def* tup_or_sig) {
 inline const Def* clos_sub_env(const Def* tup_or_sig, const Def* new_env) {
     return tup_or_sig->refine(Clos_Env_Param, new_env);
 }
+
+size_t env_idx(Defs);
+
+template<bool type>
+const Def* env_insert(World& world, Defs defs, const Def* env) {
+    size_t n = defs.size();
+    size_t x = env_idx(defs);
+
+    DefArray new_ops(n + 1);
+    for (size_t i = 0, j = 0; i != n + 1; ++i) new_ops[i] = i == x ? env : defs[j++];
+
+    return type ? world.sigma(new_ops) : world.tuple(new_ops);
+}
+
+Sigma* doms2clos(World& world, Defs doms);
+
 ///@}
 
 } // namespace thorin::clos
diff --git a/dialects/clos/phase/clos_conv.cpp b/dialects/clos/phase/clos_conv.cpp
index f13c839bac..f0289e6265 100644
--- a/dialects/clos/phase/clos_conv.cpp
+++ b/dialects/clos/phase/clos_conv.cpp
@@ -99,6 +99,30 @@ DefSet& FreeDefAna::run(Lam* lam) {
     return node->fvs;
 }
 
+/*
+ * Closure Conversion - reimpl
+ */
+
+const Def* ClosConv_::rewrite_structural(const Def* def) {
+    if (auto pi = def->isa<Pi>(); pi && pi->is_cn()) {
+        if (auto ret_pi = pi->ret_pi()) {
+            size_t n = pi->num_doms();
+            DefArray new_doms(
+                n, [&](auto i) { return i != n - 1 ? rewrite(pi->dom(i)) : world().cn(rewrite(ret_pi->dom())); });
+            return doms2clos(world(), new_doms);
+        }
+        auto new_dom = rewrite(pi->dom());
+        return doms2clos(world(), new_dom->projs());
+    } else if (auto a = match<attr>(def)) {
+        switch (a.id()) {
+            case attr::esc: break;
+        }
+    }
+    return Rewriter::rewrite_structural(def);
+}
+
+const Def* ClosConv_::rewrite_nom(Def* nom) { return Rewriter::rewrite_nom(nom); }
+
 /*
  * Closure Conversion
  */
diff --git a/dialects/clos/phase/clos_conv.h b/dialects/clos/phase/clos_conv.h
index c536883ab7..d97fcaea63 100644
--- a/dialects/clos/phase/clos_conv.h
+++ b/dialects/clos/phase/clos_conv.h
@@ -19,7 +19,7 @@ class FreeDefAna {
     FreeDefAna(World& world)
         : world_(world)
         , cur_pass_id(1)
-        , lam2nodes_(){};
+        , lam2nodes_() {}
 
     /// FreeDefAna::run will compute free defs (FD) that appear in @p lam%s body.
     /// Nominal Def%s are only considered free if they are annotated with Clos::freeBB or
@@ -73,6 +73,19 @@ class FreeDefAna {
     DefMap<std::unique_ptr<Node>> lam2nodes_;
 };
 
+class ClosConv_ : public RWPhase {
+public:
+    ClosConv_(World& world)
+        : RWPhase(world, "clos_conv")
+        , fva_(world) {}
+
+    const Def* rewrite_structural(const Def*) override;
+    const Def* rewrite_nom(Def*) override;
+
+private:
+    FreeDefAna fva_;
+};
+
 /// Performs *typed closure conversion*.
 /// This is based on the [Simply Typed Closure Conversion](https://dl.acm.org/doi/abs/10.1145/237721.237791).
 /// Closures are represented using tuples: `[Env: *, .Cn [Env, Args..], Env]`.
diff --git a/thorin/def.cpp b/thorin/def.cpp
index 471c5b001b..55d3df925b 100644
--- a/thorin/def.cpp
+++ b/thorin/def.cpp
@@ -380,7 +380,7 @@ const Def* Def::proj(nat_t a, nat_t i, const Def* dbg) const {
 
     World& w = world();
 
-    if (isa<Tuple>() || isa<Sigma>()) {
+    if (isa<Tuple, Sigma>()) {
         return op(i);
     } else if (auto arr = isa<Arr>()) {
         if (arr->arity()->isa<Top>()) return arr->body();