WCF DataContract DataMember order?

Is the xml that is created from your DataContract created in alphabetical order. I have a DataContract class defined as:

[DataContract(Name = "User", Namespace = "")]
public class User
{
    [DataMember]
    public string FirstName { get; set; }
    [DataMember]
    public string LastName { get; set; }
    [DataMember]
    public string Email { get; set; }
    [DataMember]
    public string Password { get; set; }

}

When I did the following POST:

<User>
   <FirstName>abc</FirstName>
   <LastName>123</LastName>
   <Email>email@email.com</Email>
   <Password>pass</Password>
</User>

When I did a GET after my post and returned the result as JSON, email was null, but if I POST my xml as:

 <User>
   <Email>email@email.com</Email>
   <FirstName>abc</FirstName>
   <LastName>123</LastName>
   <Password>pass</Password>
 </User>

Email is no longer null when I do a GET and return it as JSON. Why is it doing this?