The world environmental regime has encouraged nations to adopt new environmental policies and laws worldwide. But, scholars question the impact on the environment, suggesting that national policies may be "decoupled" from outcomes. We fill a gap in neoinstitutional theory by specifying the circumstances in which institutions will affect outcomes--namely, when institutions are: 1) highly structured; 2) when they penetrate actors at multiple levels of the social system; and 3) when they are persistent over time. We explore these ideas using the case of global environmentalism. Longitudinal world-level analyses find that measures of structure, penetration, and persistence are associated with lower levels of environmental degradation, as measured by global CO[2] and CFC emissions. Additionally, cross-national analyses find that penetration is associated with improved outcomes. In this case, international institutions have generated substantive social change. (Contains 1 figure, 2 tables, and 23 endnotes.)