This paper presents results from a longitudinal, qualitative study into the adoption of environmental management systems (EMS) in three companies in the UK water & sewerage industry. Based on institutional theory and the literature on EMS, four factors related to the adoption of EMS are identified: external and internal institutional forces, environmental performance issues, and economic performance issues. While previous literature has often assumed a balance of performance and institutional factors or a preponderance of performance factors, the results of this study indicate that institutional forces are the predominant drivers. The results further indicate that environmental performance issues become less important over time, whereas institutional drivers and economic performance rationales increase in importance over time. While conforming to institutional pressures can result in improved economic performance of a company, adoption of environmental management systems mostly on the basis of institutional and economic factors has wider repercussions for the state of corporate environmental management and progress towards greater ecological sustainability of business.