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Volcanic crater lakes represent the uppermost parts of magmatic-hydrothermal systems, and can be regarded as calorimeters in order to estimate subsurface heat fluxes. Here, thermal infrared images recorded by the Landsat Thematic Mapper (TM) are employed to map surface temperatures across Poás (Costa Rica), Ruapehu (New Zealand), and Kawah Ijen and Kelut (Indonesia) volcano lakes. First, lake areas are constrained by inspection of the nonthermal bands. These estimates are used to define “lake” pixels in the thermal band, and the temperature of each flagged pixel is calculated. Alternatively, the “hottest” pixel can be assumed to represent the temperature of the whole lake surface, mitigating the effects of mixed land/water pixels. Taking appropriate values for air temperature, humidity and windspeed, surface heat flux densities and power outputs are then derived from bulk aerodynamic or other formulae. Improved capabilities for monitoring crater lakes will be provided by forthcoming satellite missions, notably the Enhanced Thematic Mapper (ETM+) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER).