This paper examines how the effectiveness of regulatory framework influences levels of sulphur emissions in a scenario where, to reduce its (emission-) tax payments, a polluting firm may under-report emissions level at the risk of being audited and fined. First, a model to explain how changes in regulatory framework (e.g., audit effectiveness) and transboundary spillovers affect both actual and reported emissions is developed. Then the theoretical predictions using data for 39 European countries from 1999 to 2003 are tested and inferences about true emission levels are made. The empirical analysis supports the theoretical predictions with significant implications for the interpretation of pollution data reported to international monitoring agencies. Countries with effective regulation are likely to have relatively high reported emissions of sulphur. But this should not automatically be interpreted as weak environmental performance, because their actual pollution levels are likely to be lower than in nations with less effective regulation.