diff --git a/src/Makefile.test.include b/src/Makefile.test.include
--- a/src/Makefile.test.include
+++ b/src/Makefile.test.include
@@ -58,6 +58,7 @@
   test/mempool_tests.cpp \
   test/merkle_tests.cpp \
   test/miner_tests.cpp \
+  test/monolith_opcodes_string.cpp \
   test/multisig_tests.cpp \
   test/net_tests.cpp \
   test/netbase_tests.cpp \
diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp
--- a/src/script/interpreter.cpp
+++ b/src/script/interpreter.cpp
@@ -298,10 +298,10 @@
     if (fEnabledOpCodesMonolith) {
         switch (opcode) {
             case OP_CAT:
-                return true;
+                return false;
 
             case OP_SPLIT:
-                return true;
+                return false;
 
             case OP_BIN2NUM:
                 return true;
@@ -839,6 +839,16 @@
                     //
                     // Bitwise logic
                     //
+                    case OP_AND:
+                    case OP_OR:
+                    case OP_XOR: {
+                        if (!fEnabledOpCodesMonolith) {
+                            // Disabled opcodes.
+                            return set_error(serror,
+                                             SCRIPT_ERR_DISABLED_OPCODE);
+                        }
+                    } break;
+
                     case OP_EQUAL:
                     case OP_EQUALVERIFY:
                         // case OP_NOTEQUAL: // use OP_NUMNOTEQUAL
@@ -1238,6 +1248,65 @@
                         }
                     } break;
 
+                    //
+                    // Byte string operations
+                    //
+                    case OP_CAT: {
+                        // (x1 x2 -- out)
+                        if (stack.size() < 2) {
+                            return set_error(
+                                serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
+                        }
+                        valtype &vch1 = stacktop(-2);
+                        valtype &vch2 = stacktop(-1);
+                        if (vch1.size() + vch2.size() >
+                            MAX_SCRIPT_ELEMENT_SIZE) {
+                            return set_error(serror, SCRIPT_ERR_PUSH_SIZE);
+                        }
+                        vch1.insert(vch1.end(), vch2.begin(), vch2.end());
+                        stack.pop_back();
+                    } break;
+
+                    case OP_SPLIT: {
+                        // (in position -- x1 x2)
+                        if (stack.size() < 2) {
+                            return set_error(
+                                serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
+                        }
+                        valtype vch = stacktop(-2);
+                        int64_t nPosition =
+                            CScriptNum(stacktop(-1), fRequireMinimal).getint();
+
+                        // if nPosition is less than 0 or is larger than the
+                        // input then throw error
+                        if (nPosition < 0 ||
+                            static_cast<size_t>(nPosition) > vch.size()) {
+                            return set_error(serror,
+                                             SCRIPT_ERR_INVALID_SPLIT_RANGE);
+                        }
+
+                        stack.pop_back();
+                        stack.pop_back();
+
+                        // initialize outputs
+                        if (nPosition == 0) {
+                            stack.push_back(valtype());
+                            stack.push_back(vch);
+                        } else if (static_cast<size_t>(nPosition) ==
+                                   vch.size()) {
+                            stack.push_back(vch);
+                            stack.push_back(valtype());
+                        } else {
+                            valtype vchOut1, vchOut2;
+                            vchOut1.insert(vchOut1.end(), vch.begin(),
+                                           vch.begin() + nPosition);
+                            vchOut2.insert(vchOut2.end(),
+                                           vch.begin() + nPosition, vch.end());
+                            stack.emplace_back(move(vchOut1));
+                            stack.emplace_back(move(vchOut2));
+                        }
+                    } break;
+
                     default:
                         return set_error(serror, SCRIPT_ERR_BAD_OPCODE);
                 }
diff --git a/src/script/script_error.h b/src/script/script_error.h
--- a/src/script/script_error.h
+++ b/src/script/script_error.h
@@ -20,6 +20,9 @@
     SCRIPT_ERR_SIG_COUNT,
     SCRIPT_ERR_PUBKEY_COUNT,
 
+    /* Operands checks */
+    SCRIPT_ERR_INVALID_SPLIT_RANGE,
+
     /* Failed verify operations */
     SCRIPT_ERR_VERIFY,
     SCRIPT_ERR_EQUALVERIFY,
diff --git a/src/script/script_error.cpp b/src/script/script_error.cpp
--- a/src/script/script_error.cpp
+++ b/src/script/script_error.cpp
@@ -34,6 +34,8 @@
             return "Signature count negative or greater than pubkey count";
         case SCRIPT_ERR_PUBKEY_COUNT:
             return "Pubkey count negative or limit exceeded";
+        case SCRIPT_ERR_INVALID_SPLIT_RANGE:
+            return "Invalid OP_SPLIT range";
         case SCRIPT_ERR_BAD_OPCODE:
             return "Opcode missing or not understood";
         case SCRIPT_ERR_DISABLED_OPCODE:
diff --git a/src/test/CMakeLists.txt b/src/test/CMakeLists.txt
--- a/src/test/CMakeLists.txt
+++ b/src/test/CMakeLists.txt
@@ -35,6 +35,7 @@
 	mempool_tests.cpp
 	merkle_tests.cpp
 	miner_tests.cpp
+	monolith_opcodes_string.cpp
 	multisig_tests.cpp
 	net_tests.cpp
 	netbase_tests.cpp
diff --git a/src/test/monolith_opcodes_string.cpp b/src/test/monolith_opcodes_string.cpp
new file mode 100644