diff --git a/src/nrnoc/eion.cpp b/src/nrnoc/eion.cpp
index e8e7657b7e..6dece3efb8 100644
--- a/src/nrnoc/eion.cpp
+++ b/src/nrnoc/eion.cpp
@@ -15,9 +15,8 @@
 #include <array>
 #include <string>
 
-#include <memory>
-#include <bitset>
-#include <vector>
+#include <map>
+#include <set>
 
 #undef hoc_retpushx
 
@@ -394,17 +393,37 @@ double nrn_nernst_coef(int type) {
 /*
 It is generally an error for two models to WRITE the same concentration.
 
-This functions checks that there's no write conflict; and warns if it detects
-one. It also sets respective write flag in the style of the ion.
+nrn_check_conc_write checks that there's no write conflict; and warns if it
+detects one. It also sets respective write flag in the style of the ion.
 
 The argument `i` specifies which concentration is being written to. It's 0 for
 exterior; and 1 for interior.
 */
-void nrn_check_conc_write(Prop* p_ok, Prop* pion, int i) {
-    const int max_ions = 256;
-    static long size_;
 
-    static std::vector<std::bitset<max_ions>> chk_conc_, ion_bit_;
+// Each mechanism type index that writes a concentration has a set of ion
+// type indices it writes to.
+// ionconctype is coded as iontype*2 + i where i=1 for interior
+// So number of distinct ion mechanisms is essentially unlimited, max_int/2.
+static std::map<int, std::set<int>> mechtype2ionconctype;
+
+static void add_mechtype2ionconctype(int mechtype, int iontype, int i) {
+    auto& set_of_ionconctypes = mechtype2ionconctype[mechtype];
+    set_of_ionconctypes.insert(2 * iontype + i);  // unique though inserting for each instance.
+}
+
+static bool mech_uses_ionconctype(int mechtype, int iontype, int i) {
+    if (auto search = mechtype2ionconctype.find(mechtype); search != mechtype2ionconctype.end()) {
+        auto& set_of_ionconctypes = search->second;
+        return set_of_ionconctypes.count(2 * iontype + i) == 1;
+    }
+    return false;
+}
+
+void nrn_check_conc_write(Prop* pmech, Prop* pion, int i) {
+    // Would be less redundant to generate the "database" at
+    // mechanism registration time. But NMODL presently gives us this info
+    // only at each mechanism instance allocation.
+    add_mechtype2ionconctype(pmech->_type, pion->_type, i);
 
     Prop* p;
     int flag, j, k;
@@ -414,51 +433,32 @@ void nrn_check_conc_write(Prop* p_ok, Prop* pion, int i) {
         flag = 0400;
     }
 
-    /* Create a vector holding std::bitset to track which ions
-       are being written to the membrane */
-    if (n_memb_func > size_) {
-        chk_conc_.resize(2 * n_memb_func);
-        ion_bit_.resize(n_memb_func);
-
-        for (j = size_; j < n_memb_func; ++j) {
-            chk_conc_[2 * j].reset();
-            chk_conc_[2 * j + 1].reset();
-            ion_bit_[j].reset();
-        }
-
-        size_ = n_memb_func;
-    }
-    for (k = 0, j = 0; j < n_memb_func; ++j) {
-        if (nrn_is_ion(j)) {
-            assert(k < max_ions);
-            ion_bit_[j].reset();
-            ion_bit_[j].set(k);
-            ++k;
-        }
-    }
-
-    chk_conc_[2 * p_ok->_type + i] |= ion_bit_[pion->_type];
-    if (pion->dparam[iontype_index_dparam].get<int>() & flag) {
-        /* now comes the hard part. Is the possibility in fact actual.*/
+    auto ii = pion->dparam[iontype_index_dparam].get<int>();
+    if (ii & flag) {
+        // Is the possibility in fact actual. Unfortunately, uninserting the
+        // mechanism that writes a concentration does not reset the flag bit.
+        // So search the node property list for another mechanism that also
+        // writes this ion concentration. (that is needed anyway to
+        // fill out the warning message.)
+
+        // the pion in the node property list is before mechanisms that use the ion
         for (p = pion->next; p; p = p->next) {
-            if (p == p_ok) {
+            if (p == pmech) {
                 continue;
             }
-            auto rst = chk_conc_[2 * p->_type + i] & ion_bit_[pion->_type];
-            if (rst.any()) {
+            if (mech_uses_ionconctype(p->_type, pion->_type, i)) {
                 char buf[300];
                 Sprintf(buf,
                         "%.*s%c is being written at the same location by %s and %s",
                         (int) strlen(memb_func[pion->_type].sym->name) - 4,
                         memb_func[pion->_type].sym->name,
                         ((i == 1) ? 'i' : 'o'),
-                        memb_func[p_ok->_type].sym->name,
+                        memb_func[pmech->_type].sym->name,
                         memb_func[p->_type].sym->name);
                 hoc_warning(buf, (char*) 0);
             }
         }
     }
-    auto ii = pion->dparam[iontype_index_dparam].get<int>();
     ii |= flag;
     pion->dparam[iontype_index_dparam] = ii;
 }