The corporate environmental management tool that has undoubtedly become the most popular in recent years is environmental management systems (EMS), which often are based on the international standard ISO 14001. With few exceptions, studies on effects of EMS are relying on firm representatives’ perceived benefits, which is problematic since firm representatives often have reason to give a positive image of their employer. This study therefore aims to contribute knowledge about the environmental effects of environmental management system based on more objective environmental data. The study focuses on changes in performance over a period of 12 years and includes both firms with an EMS (66 firms) and firms without systems (50 firms). Consideration is given to the improvement efforts in the firms before EMS adoption. Analysis has been carried out using statistical methods, e.g. t-test, for six different areas: air emissions, water emissions, resource use, energy use, waste and overall environmental performance. The analysis shows that there was no statistically significant difference concerning the change in the rate of improvement between certified and non-certified firms in any of the six areas.