This paper explores the interaction between regional integration and the environment in a formal three-country, three-good model which incorporates pollution. Our main findings are: (1) whether preferential trading improves welfare depends critically on the level of domestic pollution charge extant and the direction of trade; (2) the introduction of preferential trading may lower welfare even when the pollution policy is chosen optimally; and (3) coordination of environmental policies only makes sense when pollution is transnational.