The clean development mechanism (CDM) contributes to technology transfer by financing emission reduction projects using technologies not available in the host countries. This paper provides the most comprehensive analysis of technology transfer in the CDM to-date, covering 3296 registered and proposed projects. Roughly 36% of the projects accounting for 59% of the annual emission reductions claim to involve technology transfer. Technology transfer is more common for larger projects and projects with foreign participants. Technology transfer is very heterogeneous across project types and usually involves both knowledge and equipment. As the number of projects increases, technology transfer occurs beyond the individual projects. This is observed for several of the most common project types in China and Brazil with the result that the rate of technology transfer for new projects in those countries has fallen significantly. (C) 2009 Elsevier Ltd. All rights reserved.