The aim of this article is to analyze the relationship between Environmental Management System (EMS) adoption and firms’ organizational changes. The objective is to study the organizational changes associated with EMS adoption, and to shed light on the process of change and adaptation within the firm’s internal organization, as well as between the firm and its external partners. We study organizational changes associated with EMS thanks to an unique nationwide database in France (Organizational Changes and Computerization survey of 2006). Results show that EMS adoption is associated with a tendency toward flatter hierarchies and the development of collective labor practices. They reveal a strong complementarity with the formalization of practices such as logistical arrangements and adoption of certification systems. It also involves informal modes of coordination with external partners rather than formal constraints.