Forest carbon is a new commodity to be produced and traded through market mechanisms that reduce emissions from deforestation and forest degradation (REDD). This paper examines the likely origins and effects of forest carbon through analysis of property relations associated with a REDD-like scheme in Cambodia. Two contracts in the Cardamom Mountains are compared, both implemented by an international conservation organisation with the Cambodian Forestry Administration since 2006. Although the contracts do not yet enable the sale of forest carbon, they do illustrate its processes of production, which are essentially interventions in land and forest property because they involve land-use planning, delineation of forest boundaries, and identification of rights-holders. Ethnographic examination of these processes shows how, in spite of technical standardisation, they interact with local property relations in unpredictable and paradoxical ways. The fictitious and ephemeral nature of forest carbon is therefore exposed, along with the ideals and assumptions of REDD-market proponents.