Reducing carbon emissions in the forestry sector by means of market-based schemes is considered a cost-effective measure for tackling climate change impacts. However, the transaction costs (TCs) involved are typically unknown or unquantified and therefore often neglected. In this study three types of TCs (search, design and negotiation costs) were measured in person-days and monetary terms based on a global survey of forestry carbon projects implemented across Latin-America, Asia and Africa. Cost estimates vary between zero and 1.201/tCO2 for person-days and from zero to US$ 1.738/tCO2 for monetary costs. Key drivers of TCs are identified based on the characteristics of the project in general, the transaction, the transactors involved and institutional design. The latter type of characteristic is shown to have a particularly large impact on TCs.