The ‘additionality’ criterion for the Clean Development Mechanism (CDM) (which is key to ensuring that CDM projects lead to real and additional emission reductions) has been a topic of much analysis and discussion. A number of different approaches, including those based on financial, barrier and market-penetration criteria, have been suggested as a test for additionality. A simple test for additionality is proposed that draws on the framework of the diffusion of innovations, especially the risk profile of adopters of new technologies or innovations. This approach has the potential to streamline the assessment for additionality, although it will require data on the rate of implementation of specific technologies or innovations.