<p>I created an ASMX file with a code behind file. It's working fine, but it is outputting XML.</p>

<p>However, I need it to output JSON. The ResponseFormat configuration doesn't seem to work. My code-behind is:</p>

<pre><code>[System.Web.Script.Services.ScriptService]
public class _default : System.Web.Services.WebService {
    [WebMethod]
    [ScriptMethod(UseHttpGet = true,ResponseFormat = ResponseFormat.Json)]
    public string[] UserDetails()
    {
        return new string[] { "abc", "def" };
    }
}
</code></pre>

From
WebService returns XML even when ResponseFormat set to JSON
:
Make sure that the request is a POST request, not a GET.  Scott Guthrie has a
post  explaining why
.
Though it's written specifically for jQuery, this may also be useful to you:
Using jQuery to Consume ASP.NET JSON Web Services