Changes in trade policy affect a nation's economic welfare through terms-of-trade and volume-of-trade effects. A move to global free trade would imply higher world economic welfare equal to the sum of all nations' volume-of-trade, or efficiency, effects. Since the sum of the terms-of-trade effects across all nations is zero, terms-of-trade effects are contentious. Konishi, Kowalczyk and Sjöström (2003) have shown that if customs unions do not affect trade with non-member countries, immediate global free could be achieved if free trade were proposed together with international sidepayments equal to the terms of trade effects. How large would these terms of trade effects, and hence transfers, be? This paper presents estimates from a simple computable general equilibrium model of a world economy of perfect competition. We show that, in some cases, terms-of-trade effects are small compared to efficiency gains, and transfers are not necessary for free trade. In other cases, terms-of-trade gains may account for more than 50% of a country's gains from free trade and transfers could be large. © Springer-Verlag 2008.