diff --git a/encode.c b/encode.c
index dca28e8..e56bcfe 100644
--- a/encode.c
+++ b/encode.c
@@ -66,6 +66,7 @@ opc_size(uint64_t opc)
         else
             res += 2;
     } else {
+        if (opc & OPC_LOCK) res++;
         if (opc & OPC_66) res++;
         if (opc & OPC_F2) res++;
         if (opc & OPC_F3) res++;
@@ -107,6 +108,7 @@ enc_opc(uint8_t** restrict buf, uint64_t opc)
         b2 |= (~((opc & OPC_VEXOP_MSK) >> OPC_VEXOP_IDX) & 0xf) << 3;
         *(*buf)++ = b2;
     } else {
+        if (opc & OPC_LOCK) *(*buf)++ = 0xF0;
         if (opc & OPC_66) *(*buf)++ = 0x66;
         if (opc & OPC_F2) *(*buf)++ = 0xF2;
         if (opc & OPC_F3) *(*buf)++ = 0xF3;
diff --git a/tests/test_encode.c b/tests/test_encode.c
index 0d0ce4d..d7ee42a 100644
--- a/tests/test_encode.c
+++ b/tests/test_encode.c
@@ -239,6 +239,11 @@ main(int argc, char** argv)
     TEST("\x69\x05\xf6\xff\xff\xff\x80\x00\x00\x00", FE_IMUL32rmi, FE_AX, FE_MEM(FE_IP, 0, 0, 0), 0x80);
     TEST("\x66\x69\x05\xf7\xff\xff\xff\x80\x00", FE_IMUL16rmi, FE_AX, FE_MEM(FE_IP, 0, 0, 0), 0x80);
 
+    // Test LOCK prefix
+    TEST("\xf0\x87\x08", FE_LOCK_XCHG32mr, FE_MEM(FE_AX, 0, 0, 0), FE_CX);
+    TEST("\xf0\x0f\xc1\x01", FE_LOCK_XADD32mr, FE_MEM(FE_CX, 0, 0, 0), FE_AX);
+    TEST("\x64\x67\xf0\x41\x81\x84\x00\x00\xff\xff\xff\x78\x56\x34\x12", FE_LOCK_ADD32mi|FE_ADDR32|FE_SEG(FE_FS), FE_MEM(FE_R8, 1, FE_AX, -0x100), 0x12345678);
+
     puts(failed ? "Some tests FAILED" : "All tests PASSED");
     return failed ? EXIT_FAILURE : EXIT_SUCCESS;
 }