This study presents findings from comparative research which aimed to investigate the additionality claims of Clean Development Mechanisam (CDM) afforestation projects in Tanzania, Uganda and Moldova. An “ex-post comparative baseline approach” is used to account for how project financing and background economic conditions evolve over a CDM project’s implementation and crediting periods. The paper concludes 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.