diff --git a/src/objective-c/GRPCClient/GRPCCall+OAuth2.m b/src/objective-c/GRPCClient/GRPCCall+OAuth2.m
index 83b0de18e32e45c3a150802efbc998a4ebfcc35e..cb2cee17d7754feee82f6a4babed4572c8280ae9 100644
--- a/src/objective-c/GRPCClient/GRPCCall+OAuth2.m
+++ b/src/objective-c/GRPCClient/GRPCCall+OAuth2.m
@@ -32,6 +32,7 @@
  */
 
 #import "GRPCCall+OAuth2.h"
+#import "private/GRPCRequestHeaders.h"
 
 static NSString * const kAuthorizationHeader = @"authorization";
 static NSString * const kBearerPrefix = @"Bearer ";
diff --git a/src/objective-c/GRPCClient/GRPCCall.h b/src/objective-c/GRPCClient/GRPCCall.h
index 4eda499b1a8ff4f42e5555255c96fd4aef94d15b..975ff8feffe89d6cf0caf438643468b9a0e39abd 100644
--- a/src/objective-c/GRPCClient/GRPCCall.h
+++ b/src/objective-c/GRPCClient/GRPCCall.h
@@ -48,6 +48,8 @@
 #import <Foundation/Foundation.h>
 #import <RxLibrary/GRXWriter.h>
 
+@class GRPCRequestHeaders;
+
 // Keys used in |NSError|'s |userInfo| dictionary to store the response headers and trailers sent by
 // the server.
 extern id const kGRPCHeadersKey;
@@ -70,7 +72,7 @@ extern id const kGRPCTrailersKey;
 //
 // For convenience, the property is initialized to an empty NSMutableDictionary, and the setter
 // accepts (and copies) both mutable and immutable dictionaries.
-- (NSMutableDictionary *)requestHeaders; // nonatomic
+- (GRPCRequestHeaders *)requestHeaders; // nonatomic
 - (void)setRequestHeaders:(NSDictionary *)requestHeaders; // nonatomic, copy
 
 // This dictionary is populated with the HTTP headers received from the server. This happens before
diff --git a/src/objective-c/GRPCClient/GRPCCall.m b/src/objective-c/GRPCClient/GRPCCall.m
index ff5d1c5aaff8c125a26724b3b7a3f077ebdc4080..afa01e2f0feedfe5e6f48fc4183b74d143cc8841 100644
--- a/src/objective-c/GRPCClient/GRPCCall.m
+++ b/src/objective-c/GRPCClient/GRPCCall.m
@@ -41,6 +41,7 @@
 #import "private/NSData+GRPC.h"
 #import "private/NSDictionary+GRPC.h"
 #import "private/NSError+GRPC.h"
+#import "private/GRPCRequestHeaders.h"
 
 NSString * const kGRPCHeadersKey = @"io.grpc.HeadersKey";
 NSString * const kGRPCTrailersKey = @"io.grpc.TrailersKey";
@@ -93,7 +94,7 @@ NSString * const kGRPCTrailersKey = @"io.grpc.TrailersKey";
   // the response arrives.
   GRPCCall *_retainSelf;
 
-  NSMutableDictionary *_requestHeaders;
+  GRPCRequestHeaders *_requestHeaders;
 }
 
 @synthesize state = _state;
@@ -124,19 +125,23 @@ NSString * const kGRPCTrailersKey = @"io.grpc.TrailersKey";
 
     _requestWriter = requestWriter;
 
-    _requestHeaders = [NSMutableDictionary dictionary];
+    _requestHeaders = [[GRPCRequestHeaders alloc] initWithCall:self];
   }
   return self;
 }
 
 #pragma mark Metadata
 
-- (NSMutableDictionary *)requestHeaders {
+- (GRPCRequestHeaders *)requestHeaders {
   return _requestHeaders;
 }
 
 - (void)setRequestHeaders:(NSDictionary *)requestHeaders {
-  _requestHeaders = [NSMutableDictionary dictionaryWithDictionary:requestHeaders];
+  GRPCRequestHeaders *newHeaders = [[GRPCRequestHeaders alloc] initWithCall:self];
+  for (id key in requestHeaders) {
+    newHeaders[key] = requestHeaders[key];
+  }
+  _requestHeaders = newHeaders;
 }
 
 #pragma mark Finish
@@ -230,10 +235,11 @@ NSString * const kGRPCTrailersKey = @"io.grpc.TrailersKey";
 
 #pragma mark Send headers
 
-- (void)sendHeaders:(NSDictionary *)headers {
+- (void)sendHeaders:(GRPCRequestHeaders *)headers {
   // TODO(jcanizales): Add error handlers for async failures
   [_wrappedCall startBatchWithOperations:@[[[GRPCOpSendMetadata alloc]
-                                            initWithMetadata:headers ?: @{} handler:nil]]];
+                                            initWithMetadata:[headers asDictionary] ?: @{}
+                                            handler:nil]]];
 }
 
 #pragma mark GRXWriteable implementation
diff --git a/src/objective-c/GRPCClient/private/GRPCRequestHeaders.h b/src/objective-c/GRPCClient/private/GRPCRequestHeaders.h
new file mode 100644
index 0000000000000000000000000000000000000000..320545190fa6b8e3a5f62dbaa14a28b0ae4226c5
--- /dev/null
+++ b/src/objective-c/GRPCClient/private/GRPCRequestHeaders.h
@@ -0,0 +1,51 @@
+/*
+ *
+ * Copyright 2015, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#import <Foundation/Foundation.h>
+#include <grpc/grpc.h>
+
+@class GRPCCall;
+
+@interface GRPCRequestHeaders : NSObject
+
+- (instancetype)initWithCall:(GRPCCall *)call;
+
+- (id)objectForKeyedSubscript:(NSString *)key;
+- (void)setObject:(id)obj forKeyedSubscript:(NSString *)key;
+
+- (void)removeAllObjects;
+- (void)removeObjectForKey:(NSString *)aKey;
+
+- (NSDictionary *)asDictionary;
+
+@end
\ No newline at end of file
diff --git a/src/objective-c/GRPCClient/private/GRPCRequestHeaders.m b/src/objective-c/GRPCClient/private/GRPCRequestHeaders.m
new file mode 100644
index 0000000000000000000000000000000000000000..f479ed7501cd61d68576405acd0c9eefb1deb6a7
--- /dev/null
+++ b/src/objective-c/GRPCClient/private/GRPCRequestHeaders.m
@@ -0,0 +1,128 @@
+/*
+ *
+ * Copyright 2015, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#import <Foundation/Foundation.h>
+#import "GRPCRequestHeaders.h"
+#import "GRPCCall.h"
+
+static NSString* normalizeKey(NSString* key) {
+  if ([key canBeConvertedToEncoding:NSASCIIStringEncoding]) {
+    return [key lowercaseString];
+  } else {
+    return nil;
+  }
+}
+
+static bool isKeyValuePairValid(NSString *key, id value) {
+  if ([key hasSuffix:@"-bin"]) {
+    if (![value isKindOfClass:[NSData class]]) {
+      return false;
+    }
+  } else {
+    if (![value isKindOfClass:[NSString class]]) {
+      return false;
+    }
+  }
+  return true;
+}
+
+@implementation GRPCRequestHeaders {
+  __weak GRPCCall *_call;
+  NSMutableDictionary *_proxy;
+}
+
+- (instancetype) initWithCall:(GRPCCall *)call {
+  self = [super init];
+  if (self) {
+    _call = call;
+    _proxy = [NSMutableDictionary dictionary];
+  }
+  return self;
+}
+
+- (id) objectForKeyedSubscript:(NSString *)key {
+  NSString *normalizedKey = normalizeKey(key);
+  if (normalizedKey) {
+    return _proxy[normalizedKey];
+  } else {
+    return [NSNull null];
+  }
+}
+
+- (void) setObject:(id)obj forKeyedSubscript:(NSString *)key {
+  if (_call.state == GRXWriterStateNotStarted) {
+    NSString *normalizedKey = normalizeKey(key);
+    if (normalizedKey) {
+      if (isKeyValuePairValid(key, obj)) {
+        _proxy[normalizedKey] = obj;
+      } else {
+        [NSException raise:@"Invalid key/value pair"
+                    format:@"Key %@ could not be added with value %@", key, obj];
+      }
+    } else {
+      [NSException raise:@"Invalid key" format:@"Key %@ contains illegal characters", key];
+    }
+  } else {
+    [NSException raise:@"Invalid modification"
+                format:@"Cannot modify request metadata after call is started"];
+  }
+}
+
+- (void) removeObjectForKey:(NSString *)aKey {
+  if (_call.state == GRXWriterStateNotStarted) {
+    NSString *normalizedKey = normalizeKey(aKey);
+    if (normalizedKey) {
+      [_proxy removeObjectForKey:normalizedKey];
+    } else {
+      [NSException raise:@"Invalid key" format:@"Key %@ contains illegal characters", aKey];
+    }
+  } else {
+    [NSException raise:@"Invalid modification"
+                format:@"Cannot modify request metadata after call is started"];
+  }
+}
+
+- (void) removeAllObjects {
+  if (_call.state == GRXWriterStateNotStarted) {
+    [_proxy removeAllObjects];
+  } else {
+    [NSException raise:@"Invalid modification"
+                format:@"Cannot modify request metadata after call is started"];
+  }
+}
+
+- (NSDictionary *)asDictionary {
+  return [NSDictionary dictionaryWithDictionary:_proxy];
+}
+
+@end
\ No newline at end of file