Under a customs union, countries can exchange preferential market access by coordinating external tariffs to shift profits from excluded countries. I show that the exporting rents resulting from this coordination can offset trade diversion losses produced by the union, even if its members are relatively small in world markets. Such gains come, however, at the expense of excluded countries. I show that small countries can use customs union also to foster multilateral cooperation, by increasing the incentives of excluded countries to support global free trade.