Since the Second World War, preferential trading arrangements (PTAs) have become increasingly pervasive features of the international economic system. A great deal of research has addressed the economic consequences of these arrangements, but far less effort has been made to identify the political factors leading states to enter them. In this article, the domestic political factors affecting whether countries enter PTAs are investigated, placing particular emphasis on the number of veto players within a state. It is argued that the probability of forming a PTA declines as the number of such players rises. The results, covering 194 countries from 1950 to 1999, strongly support this argument. Holding various political and economic factors constant, increasing the number of veto players within a country significantly reduces the probability of signing a PTA.