diff --git a/src/ast/euf/euf_ac_plugin.cpp b/src/ast/euf/euf_ac_plugin.cpp
index 5b4b1df66..d06db029e 100644
--- a/src/ast/euf/euf_ac_plugin.cpp
+++ b/src/ast/euf/euf_ac_plugin.cpp
@@ -109,7 +109,7 @@ namespace euf {
         m_shared_nodes.setx(n->get_id(), true, false);
         sort(monomial(m));
         m_shared_todo.insert(m_shared.size());
-        m_shared.push_back({ n, m, justification::axiom() });
+        m_shared.push_back({ n, m, justification::axiom(get_id()) });
         push_undo(is_register_shared);
     }
 
diff --git a/src/ast/euf/euf_egraph.cpp b/src/ast/euf/euf_egraph.cpp
index 12f817212..97a34a744 100644
--- a/src/ast/euf/euf_egraph.cpp
+++ b/src/ast/euf/euf_egraph.cpp
@@ -673,7 +673,7 @@ namespace euf {
         SASSERT(n1->get_root()->reaches(n1));
         SASSERT(n1->m_target);
         n1->m_target        = nullptr;
-        n1->m_justification = justification::axiom();
+        n1->m_justification = justification::axiom(null_theory_id);
         n1->get_root()->reverse_justification();
         // ---------------
         // n1 -> ... -> r1
@@ -804,7 +804,10 @@ namespace euf {
                 explain_eq(justifications, cc, a, b, j2);
         }
         else if (j.is_equality()) 
-            explain_eq(justifications, cc, j.lhs(), j.rhs());        
+            explain_eq(justifications, cc, j.lhs(), j.rhs());
+        else if (j.is_axiom() && j.theory_id() != null_theory_id) {
+            IF_VERBOSE(0, verbose_stream() << "TODO add theory axiom to justification");
+        }
         if (cc && j.is_congruence()) 
             cc->push_back(std::tuple(a->get_app(), b->get_app(), j.timestamp(), j.is_commutative()));
     }
diff --git a/src/ast/euf/euf_enode.cpp b/src/ast/euf/euf_enode.cpp
index 335c8f3e9..2149059b4 100644
--- a/src/ast/euf/euf_enode.cpp
+++ b/src/ast/euf/euf_enode.cpp
@@ -135,7 +135,7 @@ namespace euf {
         enode* prev = this;
         justification js = m_justification;
         prev->m_target = nullptr;
-        prev->m_justification = justification::axiom();
+        prev->m_justification = justification::axiom(null_theory_id);
         while (curr != nullptr) {
             enode* new_curr = curr->m_target;
             justification new_js = curr->m_justification;
diff --git a/src/ast/euf/euf_enode.h b/src/ast/euf/euf_enode.h
index 0f22ff20e..50a7ce479 100644
--- a/src/ast/euf/euf_enode.h
+++ b/src/ast/euf/euf_enode.h
@@ -36,10 +36,6 @@ namespace euf {
     typedef std::pair<enode*,bool> enode_bool_pair;
     typedef svector<enode_bool_pair> enode_bool_pair_vector;
     typedef id_var_list<> th_var_list;
-    typedef int theory_var;
-    typedef int theory_id;
-    const theory_var null_theory_var = -1;
-    const theory_id null_theory_id = -1;
 
     class enode {
         expr*         m_expr = nullptr;
diff --git a/src/ast/euf/euf_justification.h b/src/ast/euf/euf_justification.h
index f9d3b3637..faedea553 100644
--- a/src/ast/euf/euf_justification.h
+++ b/src/ast/euf/euf_justification.h
@@ -28,6 +28,11 @@ namespace euf {
 
     class enode;
 
+    typedef int theory_var;
+    typedef int theory_id;
+    const theory_var null_theory_var = -1;
+    const theory_id null_theory_id = -1;
+
     class justification {
     public:
         typedef stacked_dependency_manager<justification>             dependency_manager;
@@ -42,6 +47,7 @@ namespace euf {
         };
         kind_t m_kind;
         union {
+            int    m_theory_id;
             bool   m_comm;
             enode* m_n1;
         };
@@ -76,19 +82,27 @@ namespace euf {
             m_n2(n2)
         {}
 
-    public:
-        justification():
+        justification(int theory_id):
             m_kind(kind_t::axiom_t),
-            m_comm(false),
+            m_theory_id(theory_id),
             m_external(nullptr)
         {}
 
-        static justification axiom() { return justification(); }
+    public:
+
+        justification():
+            m_kind(kind_t::axiom_t),
+            m_theory_id(null_theory_id),
+            m_external(nullptr)
+        {}
+
+        static justification axiom(int theory_id) { return justification(theory_id); }
         static justification congruence(bool c, uint64_t ts) { return justification(c, ts); }
         static justification external(void* ext) { return justification(ext); }
         static justification dependent(dependency* d) { return justification(d, 1); }
         static justification equality(enode* a, enode* b) { return justification(a, b); }
         
+        bool   is_axiom() const { return m_kind == kind_t::axiom_t; }
         bool   is_external() const { return m_kind == kind_t::external_t; }
         bool   is_congruence() const { return m_kind == kind_t::congruence_t; }
         bool   is_commutative() const { return m_comm; }
@@ -98,6 +112,7 @@ namespace euf {
         enode* lhs() const { SASSERT(is_equality()); return m_n1; }
         enode* rhs() const { SASSERT(is_equality()); return m_n2; }
         uint64_t timestamp() const { SASSERT(is_congruence()); return m_timestamp; }
+        theory_id theory_id() const { SASSERT(is_axiom()); return m_theory_id; }
         template <typename T>
         T*  ext() const { SASSERT(is_external()); return static_cast<T*>(m_external); }            
 
@@ -106,7 +121,7 @@ namespace euf {
             case kind_t::external_t:
                 return external(copy_justification(m_external));
             case kind_t::axiom_t:
-                return axiom();
+                return axiom(m_theory_id);
             case kind_t::congruence_t:
                 return congruence(m_comm, m_timestamp);
             case kind_t::dependent_t:
@@ -114,7 +129,7 @@ namespace euf {
                 return dependent(m_dependency);
             default:
                 UNREACHABLE();
-                return axiom();
+                return axiom(-1);
             }
         }
 
diff --git a/src/ast/euf/euf_plugin.cpp b/src/ast/euf/euf_plugin.cpp
index 9551b7948..f95107bc3 100644
--- a/src/ast/euf/euf_plugin.cpp
+++ b/src/ast/euf/euf_plugin.cpp
@@ -32,7 +32,7 @@ namespace euf {
 
     void plugin::push_merge(enode* a, enode* b) {
         TRACE("plugin", tout << g.bpp(a) << " == " << g.bpp(b) << "\n");
-        g.push_merge(a, b, justification::axiom());
+        g.push_merge(a, b, justification::axiom(get_id()));
     }
 
     enode* plugin::mk(expr* e, unsigned n, enode* const* args) {