You would return a JsonResult action, MVC doesn't magically return JSON for you just because you put "?format=json" in it, at this point it's up to you to route and return the proper results.
Something along the lines of:
public ActionResult Articles(string format)
{
var articles = GetArticles();
if(!string.IsNullOrEmpty(format) && format == "json") {
return Json(articles, JsonRequestBehavior.AllowGet);
} else {
// return view?
return View(articles);
}
}