diff --git a/src/de/inetsoftware/jwebassembly/binary/BinaryModuleWriter.java b/src/de/inetsoftware/jwebassembly/binary/BinaryModuleWriter.java
index b3a3326..d96c3f3 100644
--- a/src/de/inetsoftware/jwebassembly/binary/BinaryModuleWriter.java
+++ b/src/de/inetsoftware/jwebassembly/binary/BinaryModuleWriter.java
@@ -654,6 +654,12 @@ public class BinaryModuleWriter extends ModuleWriter implements InstructionOpcod
             case d2f:
                 op = F32_DEMOTE_F64;
                 break;
+            case i2b:
+                op = I32_EXTEND8_S;
+                break;
+            case i2s:
+                op = I32_EXTEND16_S;
+                break;
             default:
                 throw new Error( "Unknown cast: " + cast );
         }
diff --git a/src/de/inetsoftware/jwebassembly/binary/InstructionOpcodes.java b/src/de/inetsoftware/jwebassembly/binary/InstructionOpcodes.java
index 8a42021..ed3a109 100644
--- a/src/de/inetsoftware/jwebassembly/binary/InstructionOpcodes.java
+++ b/src/de/inetsoftware/jwebassembly/binary/InstructionOpcodes.java
@@ -335,7 +335,19 @@ interface InstructionOpcodes {
 
     static final int F64_PROMOTE_F32        = 0xBB;
 
-    // === numerical operations ======
+    // === Sign-extension operators ====== https://github.com/WebAssembly/design/issues/1178
+
+    static final int I32_EXTEND8_S          = 0xC0;
+
+    static final int I32_EXTEND16_S         = 0xC1;
+
+    static final int I64_EXTEND8_S          = 0xC2;
+
+    static final int I64_EXTEND16_S         = 0xC3;
+
+    static final int I64_EXTEND32_S         = 0xC4;
+
+    // === ref values ====== https://github.com/WebAssembly/design/issues/1203
 
     static final int REF_NULL               = 0xD0;
 
diff --git a/src/de/inetsoftware/jwebassembly/module/ModuleGenerator.java b/src/de/inetsoftware/jwebassembly/module/ModuleGenerator.java
index 752051d..df0cbe1 100644
--- a/src/de/inetsoftware/jwebassembly/module/ModuleGenerator.java
+++ b/src/de/inetsoftware/jwebassembly/module/ModuleGenerator.java
@@ -603,20 +603,14 @@ public class ModuleGenerator {
                         instr = new WasmConvertInstruction( ValueTypeConvertion.d2f, codePos );
                         break;
                     case 145: // i2b
-                        instructions.add( new WasmConstInstruction( 24, codePos ) );
-                        instructions.add( new WasmNumericInstruction( NumericOperator.shl, ValueType.i32, codePos ) );
-                        instructions.add( new WasmConstInstruction( 24, codePos ) );
-                        instr = new WasmNumericInstruction( NumericOperator.shr_s, ValueType.i32, codePos );
+                        instr = new WasmConvertInstruction( ValueTypeConvertion.i2b, codePos );
                         break;
                     case 146: // i2c
                         instructions.add( new WasmConstInstruction( 0xFFFF, codePos ) );
                         instr = new WasmNumericInstruction( NumericOperator.and, ValueType.i32, codePos );
                         break;
                     case 147: // i2s
-                        instructions.add( new WasmConstInstruction( 16, codePos ) );
-                        instructions.add( new WasmNumericInstruction( NumericOperator.shl, ValueType.i32, codePos ) );
-                        instructions.add( new WasmConstInstruction( 16, codePos ) );
-                        instr = new WasmNumericInstruction( NumericOperator.shr_s, ValueType.i32, codePos );
+                        instr = new WasmConvertInstruction( ValueTypeConvertion.i2s, codePos );
                         break;
                     case 148: // lcmp
                         opCompare( ValueType.i64, byteCode, codePos );
diff --git a/src/de/inetsoftware/jwebassembly/module/ValueTypeConvertion.java b/src/de/inetsoftware/jwebassembly/module/ValueTypeConvertion.java
index e4e7bd1..680f68f 100644
--- a/src/de/inetsoftware/jwebassembly/module/ValueTypeConvertion.java
+++ b/src/de/inetsoftware/jwebassembly/module/ValueTypeConvertion.java
@@ -15,8 +15,6 @@
  */
 package de.inetsoftware.jwebassembly.module;
 
-import java.io.IOException;
-
 /**
  * Cast operations for converting one data type to another
  * 
@@ -36,4 +34,7 @@ public enum ValueTypeConvertion {
     d2i,
     d2l,
     d2f,
+    i2b,
+    i2c,
+    i2s,
 }
\ No newline at end of file
diff --git a/src/de/inetsoftware/jwebassembly/text/TextModuleWriter.java b/src/de/inetsoftware/jwebassembly/text/TextModuleWriter.java
index a835fe8..ea59253 100644
--- a/src/de/inetsoftware/jwebassembly/text/TextModuleWriter.java
+++ b/src/de/inetsoftware/jwebassembly/text/TextModuleWriter.java
@@ -232,6 +232,12 @@ public class TextModuleWriter extends ModuleWriter {
             case d2f:
                 op = "f32.demote/f64";
                 break;
+            case i2b:
+                op = "i32.extend8_s";
+                break;
+            case i2s:
+                op = "i32.extend16_s";
+                break;
             default:
                 throw new Error( "Unknown cast: " + cast );
         }
diff --git a/test/de/inetsoftware/jwebassembly/WasmRule.java b/test/de/inetsoftware/jwebassembly/WasmRule.java
index b52d7ed..5cc4724 100644
--- a/test/de/inetsoftware/jwebassembly/WasmRule.java
+++ b/test/de/inetsoftware/jwebassembly/WasmRule.java
@@ -257,7 +257,7 @@ public class WasmRule extends TemporaryFolder {
     private ProcessBuilder nodeJsCommand() {
         String command = System.getProperty( "node.dir" );
         command = command == null ? "node" : command + "/node";
-        return new ProcessBuilder( command, nodeScript.getAbsolutePath() );
+        return new ProcessBuilder( command, "--experimental-wasm-se", nodeScript.getAbsolutePath() );
     }
 
     /**