diff --git a/moshi/src/main/java/com/squareup/moshi/JsonAdapter.java b/moshi/src/main/java/com/squareup/moshi/JsonAdapter.java
index 1d7edda..dec374f 100644
--- a/moshi/src/main/java/com/squareup/moshi/JsonAdapter.java
+++ b/moshi/src/main/java/com/squareup/moshi/JsonAdapter.java
@@ -19,8 +19,8 @@ import java.io.IOException;
 import java.lang.reflect.AnnotatedElement;
 import java.lang.reflect.Type;
 import okio.Buffer;
+import okio.BufferedSink;
 import okio.BufferedSource;
-import okio.Sink;
 
 /**
  * Converts Java values to JSON, and JSON values to Java.
@@ -38,10 +38,9 @@ public abstract class JsonAdapter<T> {
 
   public abstract void toJson(JsonWriter writer, T value) throws IOException;
 
-  public final void toJson(Sink sink, T value) throws IOException {
+  public final void toJson(BufferedSink sink, T value) throws IOException {
     JsonWriter writer = new JsonWriter(sink);
     toJson(writer, value);
-    writer.flush();
   }
 
   public final String toJson(T value) throws IOException {
diff --git a/moshi/src/main/java/com/squareup/moshi/JsonWriter.java b/moshi/src/main/java/com/squareup/moshi/JsonWriter.java
index 7f91ef3..a8483c8 100644
--- a/moshi/src/main/java/com/squareup/moshi/JsonWriter.java
+++ b/moshi/src/main/java/com/squareup/moshi/JsonWriter.java
@@ -19,7 +19,6 @@ import java.io.Closeable;
 import java.io.Flushable;
 import java.io.IOException;
 import okio.BufferedSink;
-import okio.Okio;
 import okio.Sink;
 
 import static com.squareup.moshi.JsonScope.DANGLING_NAME;
@@ -181,11 +180,11 @@ public final class JsonWriter implements Closeable, Flushable {
   /**
    * Creates a new instance that writes a JSON-encoded stream to {@code sink}.
    */
-  public JsonWriter(Sink sink) {
+  public JsonWriter(BufferedSink sink) {
     if (sink == null) {
       throw new NullPointerException("sink == null");
     }
-    this.sink = Okio.buffer(sink);
+    this.sink = sink;
   }
 
   /**
@@ -518,13 +517,13 @@ public final class JsonWriter implements Closeable, Flushable {
         continue;
       }
       if (last < i) {
-        sink.writeUtf8(value.substring(last, i));
+        sink.writeUtf8(value, last, i);
       }
       sink.writeUtf8(replacement);
       last = i + 1;
     }
     if (last < length) {
-      sink.writeUtf8(value.substring(last, length));
+      sink.writeUtf8(value, last, length);
     }
     sink.writeByte('"');
   }
diff --git a/pom.xml b/pom.xml
index 85059c7..202fc13 100644
--- a/pom.xml
+++ b/pom.xml
@@ -26,7 +26,7 @@
     <java.version>1.7</java.version>
 
     <!-- Dependencies -->
-    <okio.version>1.1.0</okio.version>
+    <okio.version>1.4.0</okio.version>
 
     <!-- Test Dependencies -->
     <junit.version>4.12</junit.version>