jQuery $ .ajax IE8

I have a trouble getting in an responseText property of xhr object. When I'm trying to get access directly for xhr variable, I'm getting an [object Object];

$.ajax({
            'url': that.data('url'),
            'async': false,
            'type': 'get',
            'beforeSend': function() {
                onBeforeSend(that);
            },
            'success': function(res, req, xhr) {
                alert(xhr);
                alert(xhr.responseText);
                if ((typeof res == 'undefined') && (typeof xhr == 'undefined')) {
                    res = (function() {
                        try {
                            return new (this.XMLHttpRequest||ActiveXObject)('MSXML2.XMLHTTP.3.0');
                        } catch (e) {
                            alert('Exception');
                        }
                    });
                }
                container.html(res.responseText);
            },
            'error': function(res, xhr) {
                container.html(res.responseText);
            },
            'complete': function() {
                onComplete(that);
            }
        });

This trouble reveals in IE-8 with XP OS installed. Anybody help me, plesase.

try to add dataType property to the options , it can be html , JSON , xml or text