Abstract: This article analyses the extent to which national policies in the highly internationalised environmental sector are influenced by the policy preferences of political parties. The focus is on policy outputs rather than environmental performance as the central indicator of policy change. Based on a discussion of the relevant theoretical literature competing hypotheses are presented. For an empirical test, a dataset is used that includes information on the number of environmental policies adopted in 18 OECD countries at four points in time between 1970 and 2000. The results show that not only international integration, economic development and problem pressure, but also aspects of party politics, influence the number of policies adopted. The number of environmental measures increases if the governmental parties adopt more pro-environmentalist policy positions. This effect remains robust even when controlling for the institutional strength of governments, the left-right position of parties in government, the inclusion of an ecological or left-libertarian party inside the (coalition) government, and the presence of a portfolio that deals exclusively with environmental issues.