Which types of international institutions display higher ability to change states’ behavior? This article assesses the relative environmental effectiveness of a management-based (‘soft’) and an enforcement-based (‘hard’) international agreement: the UN Sofia Protocol and an EU directive. Using difference-in-differences analysis, we find that the EU directive is more effective in inducing emissions reductions than the UN’s Sofia Protocol. We propose that the EU’s enforcement capacity is a likely driver of the directive’s effectiveness. The article makes two contributions to existing literature. First, we provide causal evidence on the relative importance of overlapping international institutions in regulating environmental policy outcomes, elucidating how an apparent emissions-reducing effect of a ‘soft’ UN Protocol is in fact driven by the existence of overlapping ‘hard’ EU regulation. Second, we demonstrate how states’ enthusiasm toward emissions regulations can explain the relative effectiveness of soft and hard law institutions.