I created a class for JSON responses:

public class PostCodeJson
{
    public String Text { get; private set; }
    public String Value { get; private set; }

    #region Constructors
    /// <summary>
    /// Empty constructor
    /// </summary>
    public PostCodeJson()
    {
        this.Text = String.Empty;
        this.Value = String.Empty;
    }
    /// <summary>
    /// Constructor
    /// </summary>
    /// <param name="_text"></param>
    /// <param name="_value"></param>
    public PostCodeJson(String _text, String _value)
    {
        this.Text = _text;
        this.Value = _value;
    }
    #endregion Constructors
}

and function returns list of this class using in webservices method:

[WebMethod]

    public List<PostCodeJson> GetPostCodesCompletionListJson(String prefixText, Int32 count)
    {
        return LibDataAccess.DBServices.PostCodes.GetPostCodeJson(prefixText, count);
    }

And in aspx i do this that:

                        <script>
                            $(document).ready(function() {
                            $("#<%=pc.ClientID %>").autocomplete(
                                baseUrl + "WebServices/Autocomplete.asmx/GetPostCodesCompletionListJson",
                                {
                                    parse: function(data) {
                                        var array = new Array();
                                        for (var i = 0; i < data.length; i++) {
                                            var datum = data[i];
                                            var name = datum.Text;
                                            var display = name;
                                            array[array.length] = { data: datum, value: display, result: datum.Value };
                                        }
                                        return array;
                                    },
                                    dataType: "xml"
                            });
                        });
                        </script>

and when you enter something in the box i got an error: Request format is unrecognized for URL unexpectedly ending in '/GetPostCodesCompletionListJson

What am I doing wrong??