diff --git a/source/comp/markv_codec.cpp b/source/comp/markv_codec.cpp
index 9a080ee21..faf545fae 100644
--- a/source/comp/markv_codec.cpp
+++ b/source/comp/markv_codec.cpp
@@ -38,6 +38,7 @@
 #include "diagnostic.h"
 #include "enum_string_mapping.h"
 #include "extensions.h"
+#include "ext_inst.h"
 #include "instruction.h"
 #include "opcode.h"
 #include "operand.h"
@@ -161,6 +162,8 @@ size_t GetOperandVariableWidthChunkLength(spv_operand_type_t type) {
     case SPV_OPERAND_TYPE_OPTIONAL_MEMORY_ACCESS:
     case SPV_OPERAND_TYPE_SELECTION_CONTROL:
       return 4;
+    case SPV_OPERAND_TYPE_EXTENSION_INSTRUCTION_NUMBER:
+      return 6;
     default:
       return 0;
   }
@@ -674,6 +677,8 @@ class MarkvDecoder : public MarkvCodecBase {
   std::unordered_map<uint32_t, uint32_t> id_to_type_id_;
   // Maps a type ID to its number type description.
   std::unordered_map<uint32_t, NumberType> type_id_to_number_type_info_;
+  // Maps an ExtInstImport id to the extended instruction type.
+  std::unordered_map<uint32_t, spv_ext_inst_type_t> import_id_to_ext_inst_type_;
 };
 
 void MarkvEncoder::EncodeLiteralNumber(const Instruction& instruction,
@@ -1028,16 +1033,35 @@ spv_result_t MarkvDecoder::DecodeOperand(
         parsed_operand.type = SPV_OPERAND_TYPE_ID;
 
         if (opcode == SpvOpExtInst && parsed_operand.offset == 3) {
-          // TODO(atgoo@github.com) Work in progress.
-          assert(0 && "Not implemented");
+          // The current word is the extended instruction set id.
+          // Set the extended instruction set type for the current instruction.
+          auto ext_inst_type_iter = import_id_to_ext_inst_type_.find(id);
+          if (ext_inst_type_iter == import_id_to_ext_inst_type_.end()) {
+            return vstate_.diag(SPV_ERROR_INVALID_ID)
+                << "OpExtInst set id " << id
+                << " does not reference an OpExtInstImport result Id";
+          }
+          inst->ext_inst_type = ext_inst_type_iter->second;
         }
       }
       break;
     }
 
     case SPV_OPERAND_TYPE_EXTENSION_INSTRUCTION_NUMBER: {
-      // TODO(atgoo@github.com) Work in progress.
-      assert(0 && "Not implemented");
+      uint32_t word = 0;
+      if (!DecodeOperandWord(type, &word))
+        return vstate_.diag(SPV_ERROR_INVALID_BINARY)
+            << "Failed to read enum";
+
+      spirv_.push_back(word);
+
+      assert(SpvOpExtInst == opcode);
+      assert(inst->ext_inst_type != SPV_EXT_INST_TYPE_NONE);
+      spv_ext_inst_desc ext_inst;
+      if (grammar_.lookupExtInst(inst->ext_inst_type, word, &ext_inst))
+        return vstate_.diag(SPV_ERROR_INVALID_BINARY)
+            << "Invalid extended instruction number: " << word;
+      spvPushOperandTypes(ext_inst->operandTypes, expected_operands);
       break;
     }
 
@@ -1130,8 +1154,22 @@ spv_result_t MarkvDecoder::DecodeOperand(
       std::memcpy(&spirv_[first_word_index], str.data(), str.size());
 
       if (SpvOpExtInstImport == opcode) {
-        // TODO(atgoo@github.com) Work in progress.
-        assert(0 && "Not implemented");
+        // Record the extended instruction type for the ID for this import.
+        // There is only one string literal argument to OpExtInstImport,
+        // so it's sufficient to guard this just on the opcode.
+        const spv_ext_inst_type_t ext_inst_type =
+            spvExtInstImportTypeGet(str.data());
+        if (SPV_EXT_INST_TYPE_NONE == ext_inst_type) {
+          return vstate_.diag(SPV_ERROR_INVALID_BINARY)
+              << "Invalid extended instruction import '" << str.data() << "'";
+        }
+        // We must have parsed a valid result ID.  It's a condition
+        // of the grammar, and we only accept non-zero result Ids.
+        assert(inst->result_id);
+        const bool inserted = import_id_to_ext_inst_type_.emplace(
+            inst->result_id, ext_inst_type).second;
+        (void)inserted;
+        assert(inserted);
       }
       break;
     }
@@ -1327,7 +1365,6 @@ spv_result_t MarkvDecoder::DecodeInstruction(spv_parsed_instruction_t* inst) {
   spirv_[instruction_offset] =
       spvOpcodeMake(inst->num_words, SpvOp(inst->opcode));
 
-
   assert(inst->num_words == std::accumulate(
       parsed_operands_.begin(), parsed_operands_.end(), 1,
       [](size_t num_words, const spv_parsed_operand_t& operand) {
diff --git a/test/comp/markv_codec_test.cpp b/test/comp/markv_codec_test.cpp
index b4e029615..c43cc77ad 100644
--- a/test/comp/markv_codec_test.cpp
+++ b/test/comp/markv_codec_test.cpp
@@ -410,4 +410,24 @@ OpDecorate %1 Uniform
 )");
 }
 
+TEST(Markv, WithExtInst) {
+  TestEncodeDecode(R"(
+OpCapability Addresses
+OpCapability Kernel
+OpCapability GenericPointer
+OpCapability Linkage
+%opencl = OpExtInstImport "OpenCL.std"
+OpMemoryModel Physical32 OpenCL
+%f32 = OpTypeFloat 32
+%void = OpTypeVoid
+%void_func = OpTypeFunction %void
+%100 = OpConstant %f32 1.1
+%main = OpFunction %void None %void_func
+%entry_main = OpLabel
+%200 = OpExtInst %f32 %opencl cos %100
+OpReturn
+OpFunctionEnd
+)");
+}
+
 }  // namespace