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© by the authors, 2004. Separating and Aggregating Regime Effects page i International regimes rarely operate completely independent of each other in world politics. To capture the effect of multiple regimes, we further develop the “Oslo Potsdam solution ” to measuring regime effectiveness. In particular, we develop a methodology on how to separate and aggregate the effects in a particular policy domain, incl. institutional interaction. To demonstrate the empirical feasibility of our methodology, we show how time-series cross-sectional analysis of the regulation of sulfur emissions under the auspices of the UNECE Convention on Long-Range Transboundary Air Pollution can help us in the computation of the refined effectiveness scores