Adoption of environmental management systems constitutes one of the most important elements of corporate sustainability in recent years. The purpose of this study is to contribute knowledge as to which firms are most likely to adopt an environmental management system, those which are in greatest need of effective approaches or those which are already environmental frontrunners. A comparison between the improvement in environmental performance over a six-year period prior to ISO 14001 certification and the corresponding improvement in firms choosing not to adopt a system was performed. Environmental data was analyzed by using t-tests for six different areas: air emissions, water emissions, resource use, energy use, waste and overall environmental performance. In none of these environmental areas have we been able to find any statistically significant differences between certified and non-certified firms at 95% confidence level regarding the change in environmental performance prior to ISO 14001-certification or the corresponding period in non-certified firms. Our results indicate that it is equally likely that firms showing less or no improvement choose to implement and certify an environmental management system as it is for firms showing more improvement to do so.