Although the theoretical literature on the performance of voluntary approaches to environmental protection has progressed quite far in the last decade, no one has rigorously addressed the obvious point that even voluntary emissions control policies must be enforced. This paper examines the consequences of the need for costly enforcement of voluntary environmental agreements with industries on the ability of these agreements to meet regulatory objectives, the levels of industry participation with these agreements, and the relative efficiency of voluntary and regulatory approaches. We find that enforcement costs that are borne by the members of a voluntary emissions control agreement limit the circumstances under which an agreement can form i...