This paper explores tax policy effects and revenue implications of joining the World Trade Organization (WTO) and its predecessor, the General Agreement on Tariffs and Trade (GATT). It documents that countries joining GATT/WTO after 1990 have implemented tariff-cum-tax reforms, lowering tariff rates and raising consumption tax rates, in particular through reform or introduction of a value added tax (VAT). Employing a panel of 97 developing and transitional countries, 31 of which joined GATT/WTO between 1990 and 2011, using robust difference-in-difference specifications as well as the synthetic control method, we find a statistically and economically significant decline in revenues from import duties. This finding supports concerns about revenue losses, but also corroborates the efficacy of the late Uruguay GATT and the WTO trade regimes in promoting free trade among new members. Regarding consumption taxes, we find robust evidence that revenue substitution was successful, since revenue losses from import duties were more than compensated for by enhanced revenues from consumption taxes. With regard to the timing of the revenue effects, our results show that revenue losses in import duties mostly take place at the time of membership or later. Changes in consumption taxation, however, exhibit pre-membership effects, as revenues are increased, and VAT is adopted, often a few years ahead of losses in import duties. No such effects are found before the start of the accession negotiations, indicating that consumption tax reforms are initiated once a country is on the road to GATT/WTO membership.