This paper integrates a stakeholder perspective into the resource-based view of the firm, to analyze the mechanisms that link the adoption of the international Environmental Management Standard ISO 14001 to firms' competitive advantage. This paper shows that the perceived competitiveness impact of the standard depends mostly on the involvement of firms' external stakeholders (distributors, customers, community members, and regulatory agencies) in its design. ISO 14001 is a process standard, and it is difficult for stakeholders to get credible information on the effectiveness of the standard if they are not involved in its design. Stakeholders' involvement in a firm's ISO 14001 standard becomes a valuable organizational capability, which is difficult to imitate by competitors. The analysis is supported by primary data collected from a questionnaire mailed to 152 firms, resulting in 55 observations representing 30% of the total number of firms certified in the U.S. in August 1998.