diff --git a/okhttp-tests/src/test/java/com/squareup/okhttp/FormEncodingBuilderTest.java b/okhttp-tests/src/test/java/com/squareup/okhttp/FormEncodingBuilderTest.java
index a9533bf78..8c2eca817 100644
--- a/okhttp-tests/src/test/java/com/squareup/okhttp/FormEncodingBuilderTest.java
+++ b/okhttp-tests/src/test/java/com/squareup/okhttp/FormEncodingBuilderTest.java
@@ -25,11 +25,12 @@ public final class FormEncodingBuilderTest {
     RequestBody formEncoding = new FormEncodingBuilder()
         .add("a&b", "c=d")
         .add("space, the", "final frontier")
+        .add("%25", "%25")
         .build();
 
     assertEquals("application/x-www-form-urlencoded", formEncoding.contentType().toString());
 
-    String expected = "a%26b=c%3Dd&space%2C+the=final+frontier";
+    String expected = "a%26b=c%3Dd&space,%20the=final%20frontier&%2525=%2525";
     assertEquals(expected.length(), formEncoding.contentLength());
 
     Buffer out = new Buffer();
@@ -37,6 +38,19 @@ public final class FormEncodingBuilderTest {
     assertEquals(expected, out.readUtf8());
   }
 
+  @Test public void addEncoded() throws Exception {
+    RequestBody formEncoding = new FormEncodingBuilder()
+        .addEncoded("a+=& b", "c+=& d")
+        .addEncoded("e+=& f", "g+=& h")
+        .addEncoded("%25", "%25")
+        .build();
+
+    String expected = "a%20%3D%26%20b=c%20%3D%26%20d&e%20%3D%26%20f=g%20%3D%26%20h&%25=%25";
+    Buffer out = new Buffer();
+    formEncoding.writeTo(out);
+    assertEquals(expected, out.readUtf8());
+  }
+
   @Test public void encodedPair() throws Exception {
     RequestBody formEncoding = new FormEncodingBuilder()
         .add("sim", "ple")
diff --git a/okhttp/src/main/java/com/squareup/okhttp/FormEncodingBuilder.java b/okhttp/src/main/java/com/squareup/okhttp/FormEncodingBuilder.java
index 63eac1ab9..8a253dafc 100644
--- a/okhttp/src/main/java/com/squareup/okhttp/FormEncodingBuilder.java
+++ b/okhttp/src/main/java/com/squareup/okhttp/FormEncodingBuilder.java
@@ -15,8 +15,6 @@
  */
 package com.squareup.okhttp;
 
-import java.io.UnsupportedEncodingException;
-import java.net.URLEncoder;
 import okio.Buffer;
 
 /**
@@ -34,13 +32,24 @@ public final class FormEncodingBuilder {
     if (content.size() > 0) {
       content.writeByte('&');
     }
-    try {
-      content.writeUtf8(URLEncoder.encode(name, "UTF-8"));
-      content.writeByte('=');
-      content.writeUtf8(URLEncoder.encode(value, "UTF-8"));
-    } catch (UnsupportedEncodingException e) {
-      throw new AssertionError(e);
+    HttpUrl.canonicalize(content, name, 0, name.length(),
+        HttpUrl.QUERY_COMPONENT_ENCODE_SET, false, true);
+    content.writeByte('=');
+    HttpUrl.canonicalize(content, value, 0, value.length(),
+        HttpUrl.QUERY_COMPONENT_ENCODE_SET, false, true);
+    return this;
+  }
+
+  /** Add new key-value pair. */
+  public FormEncodingBuilder addEncoded(String name, String value) {
+    if (content.size() > 0) {
+      content.writeByte('&');
     }
+    HttpUrl.canonicalize(content, name, 0, name.length(),
+        HttpUrl.QUERY_COMPONENT_ENCODE_SET, true, true);
+    content.writeByte('=');
+    HttpUrl.canonicalize(content, value, 0, value.length(),
+        HttpUrl.QUERY_COMPONENT_ENCODE_SET, true, true);
     return this;
   }
 
diff --git a/okhttp/src/main/java/com/squareup/okhttp/HttpUrl.java b/okhttp/src/main/java/com/squareup/okhttp/HttpUrl.java
index 2cda2c47f..709372d69 100644
--- a/okhttp/src/main/java/com/squareup/okhttp/HttpUrl.java
+++ b/okhttp/src/main/java/com/squareup/okhttp/HttpUrl.java
@@ -255,12 +255,12 @@ import okio.Buffer;
 public final class HttpUrl {
   private static final char[] HEX_DIGITS =
       { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F' };
-  private static final String USERNAME_ENCODE_SET = " \"':;<=>@[]^`{}|/\\?#";
-  private static final String PASSWORD_ENCODE_SET = " \"':;<=>@[]\\^`{}|/\\?#";
-  private static final String PATH_SEGMENT_ENCODE_SET = " \"<>^`{}|/\\?#";
-  private static final String QUERY_ENCODE_SET = " \"'<>#";
-  private static final String QUERY_COMPONENT_ENCODE_SET = " \"'<>#&=";
-  private static final String FRAGMENT_ENCODE_SET = "";
+  static final String USERNAME_ENCODE_SET = " \"':;<=>@[]^`{}|/\\?#";
+  static final String PASSWORD_ENCODE_SET = " \"':;<=>@[]\\^`{}|/\\?#";
+  static final String PATH_SEGMENT_ENCODE_SET = " \"<>^`{}|/\\?#";
+  static final String QUERY_ENCODE_SET = " \"'<>#";
+  static final String QUERY_COMPONENT_ENCODE_SET = " \"'<>#&=";
+  static final String FRAGMENT_ENCODE_SET = "";
 
   /** Either "http" or "https". */
   private final String scheme;
@@ -1422,10 +1422,10 @@ public final class HttpUrl {
           || (codePoint == '%' && !alreadyEncoded)
           || (query && codePoint == '+')) {
         // Slow path: the character at i requires encoding!
-        StringBuilder out = new StringBuilder();
-        out.append(input, pos, i);
+        Buffer out = new Buffer();
+        out.writeUtf8(input, pos, i);
         canonicalize(out, input, i, limit, encodeSet, alreadyEncoded, query);
-        return out.toString();
+        return out.readUtf8();
       }
     }
 
@@ -1433,7 +1433,7 @@ public final class HttpUrl {
     return input.substring(pos, limit);
   }
 
-  static void canonicalize(StringBuilder out, String input, int pos, int limit,
+  static void canonicalize(Buffer out, String input, int pos, int limit,
       String encodeSet, boolean alreadyEncoded, boolean query) {
     Buffer utf8Buffer = null; // Lazily allocated.
     int codePoint;
@@ -1444,7 +1444,7 @@ public final class HttpUrl {
         // Skip this character.
       } else if (query && codePoint == '+') {
         // HTML permits space to be encoded as '+'. We use '%20' to avoid special cases.
-        out.append(alreadyEncoded ? "%20" : "%2B");
+        out.writeUtf8(alreadyEncoded ? "%20" : "%2B");
       } else if (codePoint < 0x20
           || codePoint >= 0x7f
           || encodeSet.indexOf(codePoint) != -1
@@ -1456,13 +1456,13 @@ public final class HttpUrl {
         utf8Buffer.writeUtf8CodePoint(codePoint);
         while (!utf8Buffer.exhausted()) {
           int b = utf8Buffer.readByte() & 0xff;
-          out.append('%');
-          out.append(HEX_DIGITS[(b >> 4) & 0xf]);
-          out.append(HEX_DIGITS[b & 0xf]);
+          out.writeByte('%');
+          out.writeByte(HEX_DIGITS[(b >> 4) & 0xf]);
+          out.writeByte(HEX_DIGITS[b & 0xf]);
         }
       } else {
         // This character doesn't need encoding. Just copy it over.
-        out.append((char) codePoint);
+        out.writeUtf8CodePoint(codePoint);
       }
     }
   }