Fossil fuel subsidy reform has in recent years been addressed by international economic organizations including the International Monetary Fund (IMF) and the Organisation for Economic Co-operation and Development (OECD). The two organizations have differed significantly in how they define fossil fuel subsidies. The IMF's definition constitutes a radical break with previous definitions by including environmental externalities, while the OECD's is more conventional. The article explores the factors that explain why these international economic organizations have approached fossil fuel subsidies so differently. The exact definition of fossil fuel subsidies is contested. Furthermore, fossil fuels subsidies can be framed in ways that emphasize, respectively, their macroeconomic, fiscal, environmental, and distributive consequences. The article finds that institutional interaction lifted OECD involvement in fossil fuel subsidies to a new level, whereas the impetus to address fossil fuel subsidies within the IMF came largely from the IMF staff. In both cases, the organization's bureaucracy constituted the most important factor shaping how the organizations addressed such subsidies and hence the main reason why they differ in how they approach fossil fuel subsidies.