This article examines the institutional impact of environmental management systems (EMSs), focusing on ISO 14001. It develops a pluralistic framework for thinking about the dynamic of corporate self-regulation that we term the polyphonic model. It argues that the adoption of ISO 14001 can move the firm into a new equilibrium trajectory, which enmeshes together environmental and economic goals and reflects greater sensitivity to ecological concerns. There is a positive reciprocal cycle between the pro-environmental structural changes induced by ISO 14001 and the employees' attitudes toward the firm and the environment. In order to examine ISO 14001 institutional impact, we conducted a series of interviews with managers and administered questionnaires to employees in 24 Israeli firms with and without certification. The findings indicate that the perceived environmental commitment of certified firms was higher than that of noncertified firms and was higher among employees that perceived the EMS as more highly integrated in the firm. Perceptions of the standard's integration were also found to be positively correlated with personal environmental commitment. The results also indicate that the increase in the firm's environmental commitment was positively associated with employees' organizational citizenship behavior within certified firms. Further indications of the pro-environmental dynamic induced by ISO 14001 were found in the in-depth interviews.