Deliberation incompatibilism is the view that an agent being rational and deliberating about which of (mutually excluding) actions to perform, is incompatible with her believing that there exist prior conditions that render impossible the performance of either one of these actions. However, the main argument for this view, associated most prominently with Peter van Inwagen, appears to have been widely rejected by contemporary authors on free will. In this paper I argue first that a closer examination of van Inwagen's argument shows that the standard objections are based on a misunderstanding of the notion of ‘deliberation’ presupposed in this argument. Second, I attempt to strengthen the case for deliberation incompatibilism by offering a different argument in its support.