public class MessageFormatter : IDispatchMessageFormatter
    {
        private const string CONTENT_TYPE_XML = "text/xml";
        private const string CONTENT_TYPE_JSON = "application/Json";
        private readonly IDispatchMessageFormatter originalFormatter;

public MessageFormatter(IDispatchMessageFormatter dispatchMessageFormatter)
        {
            this.originalFormatter = dispatchMessageFormatter;
        }

#region IDispatchMessageFormatter
        public void DeserializeRequest(Message message, object[] parameters)
        {
            this.originalFormatter.DeserializeRequest(message,parameters);
        }

public Message SerializeReply(MessageVersion messageVersion, object[] parameters, object result)
        {
        }

#endregion
    }

string alt = WebOperationContext.Current.IncomingRequest.UriTemplateMatch.QueryParameters["alt"];
if(alt=="xml")
{
   WebOperationContext.Current.OutgoingResponse.Format = WebMessageFormat.Xml;
   WebOperationContext.Current.OutgoingResponse.ContentType = CONTENT_TYPE_XML;
}

if(alt=="json")
{
   WebOperationContext.Current.OutgoingResponse.Format = WebMessageFormat.Json;
   WebOperationContext.Current.OutgoingResponse.ContentType = CONTENT_TYPE_JSON;
}

public class MessageFormatterEndpointBehavior : IEndpointBehavior
    {
        #region IEndpointBehavior Members
        public void AddBindingParameters(ServiceEndpoint endpoint, System.ServiceModel.Channels.BindingParameterCollection bindingParameters)
        {
        }

public void ApplyClientBehavior(ServiceEndpoint endpoint, ClientRuntime clientRuntime)
        {
        }

public void ApplyDispatchBehavior(ServiceEndpoint endpoint, EndpointDispatcher endpointDispatcher)
        {
            foreach (OperationDescription operation in endpoint.Contract.Operations)
            {
                operation.Formatter = new MessageFormatter(operation.Formatter);
            }
        }

public void Validate(ServiceEndpoint endpoint)
        {
        }
        #endregion
    }

public class MessageFormatterEndpointBehaviorExtensionElement : BehaviorExtensionElement
    {
        public override Type BehaviorType
        {
            get { return typeof(MessageFormatterEndpointBehavior); }
        }

protected override object CreateBehavior()
        {
            return new MessageFormatterEndpointBehavior();
        }
    }

<system.serviceModel>
    <bindings>
      <webHttpBinding>
        <binding name="webBinding">
        </binding>
      </webHttpBinding>
    </bindings>
    <services>
      <service name="WCFMessageFormatter.Services.TankService" behaviorConfiguration="testServiceBehavior">
        <endpoint address="http://localhost:8080/TankService" behaviorConfiguration="webBehavior"
                  binding="webHttpBinding" bindingConfiguration="webBinding" contract="WCFMessageFormatter.Contracts.ServiceContracts.ITankService">
        </endpoint>
      </service>
    </services>
    <behaviors>
      <endpointBehaviors>
        <behavior name="webBehavior">
          <webHttp />
          <endpointMessageFormatter />
        </behavior>
      </endpointBehaviors>
      <serviceBehaviors>
        <behavior name="testServiceBehavior">
        </behavior>
      </serviceBehaviors>
    </behaviors>
    <extensions>
      <behaviorExtensions>
        <add
          name="endpointMessageFormatter"
          type="WCFMessageFormatter.CustomServiceBehaviors.MessageFormatterEndpointBehaviorExtensionElement, WCFMessageFormatter.CustomServiceBehaviors, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"
          />
      </behaviorExtensions>
    </extensions>
  </system.serviceModel>
  <system.webServer>
    <modules runAllManagedModulesForAllRequests="true">
      <add name="UrlRoutingModule" type="System.Web.Routing.UrlRoutingModule,System.Web.Routing,Version=4.0.0.0,Culture=neutral,PublicKeyToken=31BF3856AD364E35" />
    </modules>
  </system.webServer>
</configuration>