We analyse the welfare effects of environmental policy arising from the formation of an international environmental agreement on the participating and non-participating countries and thus shed light on the potential incentives for a country to join such an agreement. Within a N-country Q-goods general equilibrium framework under free-trade conditions, we consider unilateral and cooperative policy settings and, within the latter, country-specific and fully harmonized policies within the agreement. A key result of the paper is the emergence of a negative relationship, arising from terms of trade effects, between the welfare changes of the participating and non-participating countries following the formation of the agreement. These however do not result in a zero sum welfare outcome for the world as a whole.