diff --git a/jme/src/main/java/org/luaj/vm2/libs/jme/JmeIoLib.java b/jme/src/main/java/org/luaj/vm2/libs/jme/JmeIoLib.java
index 8c60a940..de6c8501 100644
--- a/jme/src/main/java/org/luaj/vm2/libs/jme/JmeIoLib.java
+++ b/jme/src/main/java/org/luaj/vm2/libs/jme/JmeIoLib.java
@@ -83,20 +83,15 @@ public class JmeIoLib extends IoLib {
 	}
 	
 	protected File openFile( String filename, boolean readMode, boolean appendMode, boolean updateMode, boolean binaryMode ) throws IOException {
+		if ( appendMode || updateMode ) {
+			throw new IOException("unsupported mode");
+		}
 		String url = "file:///" + filename;
 		int mode  = readMode? Connector.READ: Connector.READ_WRITE;
 		StreamConnection conn = (StreamConnection) Connector.open( url, mode );
 		File f = readMode? 
 				new FileImpl(conn, conn.openInputStream(), null):
-				new FileImpl(conn, conn.openInputStream(), conn.openOutputStream());
-		/*
-		if ( appendMode ) {
-			f.seek("end",0);
-		} else {
-			if ( ! readMode )
-				conn.truncate(0);
-		}
-		*/
+				new FileImpl(conn, null, conn.openOutputStream());
 		return f;
 	}
 	
diff --git a/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib$FileImpl.class b/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib$FileImpl.class
index 736d070c..c6e4f505 100644
Binary files a/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib$FileImpl.class and b/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib$FileImpl.class differ
diff --git a/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib$StdinFile.class b/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib$StdinFile.class
index de1b21f6..69eff34e 100644
Binary files a/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib$StdinFile.class and b/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib$StdinFile.class differ
diff --git a/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib$StdoutFile.class b/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib$StdoutFile.class
index 70ecf29d..6f3e1d23 100644
Binary files a/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib$StdoutFile.class and b/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib$StdoutFile.class differ
diff --git a/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib.class b/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib.class
index a0999627..53a6d00c 100644
Binary files a/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib.class and b/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib.class differ
diff --git a/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib.java b/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib.java
index 4ced4734..cfd5dc5f 100644
--- a/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib.java
+++ b/jse/src/main/java/org/luaj/vm2/libs/jse/JseIoLib.java
@@ -84,14 +84,14 @@ public class JseIoLib extends IoLib {
 	}
 	
 	protected File openFile( String filename, boolean readMode, boolean appendMode, boolean updateMode, boolean binaryMode ) throws IOException {
-		RandomAccessFile f = new RandomAccessFile(filename,readMode? "r": "rw");
+		RandomAccessFile f = new RandomAccessFile(filename, readMode && !updateMode ? "r": "rw");
 		if ( appendMode ) {
 			f.seek(f.length());
 		} else {
 			if ( ! readMode )
 				f.setLength(0);
 		}
-		return new FileImpl( f );
+		return new FileImpl( f, readMode || updateMode, !readMode || updateMode, appendMode );
 	}
 	
 	protected File openProgram(String prog, String mode) throws IOException {
@@ -104,7 +104,7 @@ public class JseIoLib extends IoLib {
 	protected File tmpFile() throws IOException {
 		java.io.File f = java.io.File.createTempFile(".luaj","bin");
 		f.deleteOnExit();
-		return new FileImpl( new RandomAccessFile(f,"rw") );
+		return new FileImpl( new RandomAccessFile(f,"rw"), true, true, false );
 	}
 	
 	private static void notimplemented() {
@@ -116,21 +116,27 @@ public class JseIoLib extends IoLib {
 		private final RandomAccessFile file;
 		private final InputStream is;
 		private final OutputStream os;
+		private final boolean readable;
+		private final boolean writable;
+		private final boolean append;
 		private boolean closed = false;
 		private boolean nobuffer = false;
-		private FileImpl( RandomAccessFile file, InputStream is, OutputStream os ) {
+		private FileImpl( RandomAccessFile file, InputStream is, OutputStream os, boolean readable, boolean writable, boolean append ) {
 			this.file = file;
 			this.is = is!=null? is.markSupported()? is: new BufferedInputStream(is): null;
 			this.os = os;
+			this.readable = readable;
+			this.writable = writable;
+			this.append = append;
 		}
-		private FileImpl( RandomAccessFile f ) {
-			this( f, null, null );
+		private FileImpl( RandomAccessFile f, boolean readable, boolean writable, boolean append ) {
+			this( f, null, null, readable, writable, append );
 		}
 		private FileImpl( InputStream i ) {
-			this( null, i, null );
+			this( null, i, null, true, false, false );
 		}
 		private FileImpl( OutputStream o ) {
-			this( null, null, o );
+			this( null, null, o, false, true, false );
 		}
 		public String tojstring() {
 			return "file (" + (this.closed ? "closed" : String.valueOf(this.hashCode())) + ")";
@@ -149,11 +155,15 @@ public class JseIoLib extends IoLib {
 				os.flush();
 		}
 		public void write(LuaString s) throws IOException {
+			if ( ! writable )
+				throw new IOException("file is not writable");
 			if ( os != null )
 				os.write( s.m_bytes, s.m_offset, s.m_length );
-			else if ( file != null )
+			else if ( file != null ) {
+				if ( append )
+					file.seek(file.length());
 				file.write( s.m_bytes, s.m_offset, s.m_length );
-			else
+			} else
 				notimplemented();
 			if ( nobuffer )
 				flush();
@@ -181,11 +191,15 @@ public class JseIoLib extends IoLib {
 
 		// get length remaining to read
 		public int remaining() throws IOException {
+			if ( ! readable )
+				throw new IOException("file is not readable");
 			return file!=null? (int) (file.length()-file.getFilePointer()): -1;
 		}
 		
 		// peek ahead one character
 		public int peek() throws IOException {
+			if ( ! readable )
+				throw new IOException("file is not readable");
 			if ( is != null ) {
 				is.mark(1);
 				int c = is.read();
@@ -203,6 +217,8 @@ public class JseIoLib extends IoLib {
 		
 		// return char if read, -1 if eof, throw IOException on other exception
 		public int read() throws IOException {
+			if ( ! readable )
+				throw new IOException("file is not readable");
 			if ( is != null )
 				return is.read();
 			else if ( file != null ) {
@@ -214,6 +230,8 @@ public class JseIoLib extends IoLib {
 
 		// return number of bytes read if positive, -1 if eof, throws IOException
 		public int read(byte[] bytes, int offset, int length) throws IOException {
+			if ( ! readable )
+				throw new IOException("file is not readable");
 			if (file!=null) {
 				return file.read(bytes, offset, length);
 			} else if (is!=null) {
diff --git a/jse/src/test/java/org/luaj/vm2/FragmentsTest.java b/jse/src/test/java/org/luaj/vm2/FragmentsTest.java
index c730a143..c491c2cc 100644
--- a/jse/src/test/java/org/luaj/vm2/FragmentsTest.java
+++ b/jse/src/test/java/org/luaj/vm2/FragmentsTest.java
@@ -21,6 +21,9 @@
  ******************************************************************************/
 package org.luaj.vm2;
 
+import java.io.File;
+import java.io.FileOutputStream;
+import java.io.IOException;
 import java.io.Reader;
 import java.io.StringReader;
 import java.util.concurrent.atomic.AtomicReference;
@@ -236,6 +239,98 @@ public class FragmentsTest extends TestSuite {
 			runFragment(LuaValue.valueOf(7), "return 1 | 2 & 6\n");
 		}
 
+		public void testIoOpenReadModeDisallowsWrite() throws Exception {
+			File file = writeTempFile("read-mode", "hello");
+			try {
+				Globals globals = JsePlatform.standardGlobals();
+				try {
+					globals.load("local f = io.open(" + quote(file.getAbsolutePath()) + ", 'r') f:write('x')", "io_r.lua").call();
+					fail("expected write on read-only file to fail");
+				} catch (LuaError e) {
+					assertTrue(e.getMessage().indexOf("not writable") >= 0);
+				}
+			} finally {
+				file.delete();
+			}
+		}
+
+		public void testIoOpenWriteModeDisallowsRead() throws Exception {
+			File file = File.createTempFile("luaj-io", ".txt");
+			try {
+				Globals globals = JsePlatform.standardGlobals();
+				try {
+					globals.load("local f = io.open(" + quote(file.getAbsolutePath()) + ", 'w') return f:read('*a')", "io_w.lua").call();
+					fail("expected read on write-only file to fail");
+				} catch (LuaError e) {
+					assertTrue(e.getMessage().indexOf("not readable") >= 0);
+				}
+			} finally {
+				file.delete();
+			}
+		}
+
+		public void testIoOpenUpdateModesSupportReadWrite() throws Exception {
+			File file = writeTempFile("update-mode", "abc");
+			try {
+				Globals globals = JsePlatform.standardGlobals();
+				Varargs result = globals.load(
+					"local f = assert(io.open(" + quote(file.getAbsolutePath()) + ", 'r+'))\n" +
+					"local before = f:read('*a')\n" +
+					"f:seek('set', 0)\n" +
+					"f:write('xyz')\n" +
+					"f:seek('set', 0)\n" +
+					"local after = f:read('*a')\n" +
+					"f:close()\n" +
+					"return before, after\n",
+					"io_rplus.lua").invoke();
+				assertEquals("abc", result.arg1().tojstring());
+				assertEquals("xyz", result.arg(2).tojstring());
+			} finally {
+				file.delete();
+			}
+		}
+
+		public void testIoOpenAppendPreservesExistingContent() throws Exception {
+			File file = writeTempFile("append-mode", "abc");
+			try {
+				Globals globals = JsePlatform.standardGlobals();
+				LuaValue result = globals.load(
+					"local f = assert(io.open(" + quote(file.getAbsolutePath()) + ", 'a'))\n" +
+					"f:write('xyz')\n" +
+					"f:close()\n" +
+					"local g = assert(io.open(" + quote(file.getAbsolutePath()) + ", 'r'))\n" +
+					"local contents = g:read('*a')\n" +
+					"g:close()\n" +
+					"return contents\n",
+					"io_a.lua").call();
+				assertEquals("abcxyz", result.tojstring());
+			} finally {
+				file.delete();
+			}
+		}
+
+		public void testIoOpenAppendUpdateAppendsOnWriteAfterSeek() throws Exception {
+			File file = writeTempFile("append-update", "abc");
+			try {
+				Globals globals = JsePlatform.standardGlobals();
+				Varargs result = globals.load(
+					"local f = assert(io.open(" + quote(file.getAbsolutePath()) + ", 'a+'))\n" +
+					"f:seek('set', 0)\n" +
+					"local before = f:read('*a')\n" +
+					"f:seek('set', 0)\n" +
+					"f:write('xyz')\n" +
+					"f:seek('set', 0)\n" +
+					"local after = f:read('*a')\n" +
+					"f:close()\n" +
+					"return before, after\n",
+					"io_aplus.lua").invoke();
+				assertEquals("abc", result.arg1().tojstring());
+				assertEquals("abcxyz", result.arg(2).tojstring());
+			} finally {
+				file.delete();
+			}
+		}
+
 		public void testTableMove() {
 			runFragment(
 				LuaValue.varargsOf(new LuaValue[] {
@@ -844,5 +939,20 @@ public class FragmentsTest extends TestSuite {
 					+ "return v1, v2, v3");
 			
 		}
+
+		private static File writeTempFile(String prefix, String contents) throws IOException {
+			File file = File.createTempFile(prefix, ".txt");
+			FileOutputStream out = new FileOutputStream(file);
+			try {
+				out.write(contents.getBytes("UTF-8"));
+			} finally {
+				out.close();
+			}
+			return file;
+		}
+
+		private static String quote(String value) {
+			return "'" + value.replace("\\", "\\\\").replace("'", "\\'") + "'";
+		}
 	}
 }