The Clean Development Mechanism (CDM) is a prominent example of the contemporary turn towards more hybrid modes of global environmental governance. It epitomizes the trend away from hierarchical state regulation towards softer forms of steering along the public-private frontier. In this article we analyze the legitimacy of this novel governance arrangement. While we approach input legitimacy as a procedural ideal that guarantees actors affected by a CDM project voice in the project design and implementation, we relate output legitimacy to the effectiveness or problem solving capacity of the CDM institutions. In contrast to the mainstream understanding of the CDM as a policy mechanism that will secure both goals at the same time and thus reduce the legitimacy gap in global environmental governance, our study points to central trade-offs between the procedural quality and the effectiveness of the CDM project cycle. These trade-offs are illustrated by three carbon projects in Chile, China and Mexico and raise questions for the continued study of legitimacy in global environmental governance.