The last few decades have been marked by the emergence of a number of environmental protection regimes in the international arena. We know little about the effectiveness of these regimes, however, largely because such evaluations face the formidable obstacles of a scarcity of time series data on environmental quality, a complex mix of nonpolicy factors that affect environmental quality, and the fact that participation in nearly all international environmental agreements is voluntary, which means that policy effectiveness must be estimated from self-selected samples. In this article we assess the effects of the 1985 Helsinki Protocol for reducing sulfur dioxide emissions in Europe, paying particularly close attention to the obstacles noted above. We find that while nations ratifying the Helsinki Protocol have experienced significant emission reductions, the protocol itself has had no discernible effect on emissions. We end the article by discussing the implication of these results for the effectiveness of international environmental regimes in general.