This study presents findings from a systematic comparative research effort to investigate the additionality claims of CDM afforestation projects in Tanzania, Uganda and Moldova. Using what we refer to as an ex-post comparative baseline approach that accounts for how project financing and background economic conditions evolve over a CDM project’s implementation and crediting periods, we demonstrate that the projects in Uganda and Moldova are very likely to be fully additional while only approximately one-quarter of carbon credits resulting from the Tanzania project are genuine. The conditions of additionality can change significantly over the course of a CDM project in a way that undermines project environmental integrity because the CDM rules do not accommodate changing baseline conditions. Rather, current CDM rules allow initial baseline conditions to be frozen over a project’s crediting period. We recommend that a reformed CDM, REDD, NAMA or other new market mechanism adopt some of the elements of our approach including use of comparative performance benchmarks, an additionality risk management tool and engaging donors in the development of “ODAbaselines” for climate mitigation projects which combine carbon finance and development assistance.