Grimsvotn is located beneath Vatnajokull, Europe's largest temperate ice cap. As a part of ongoing research on heat flux, morphological changes and volcanic processes at Grimsvotn, Airborne Thematic Mapper (ATM) imagery and aerial photographs were acquired in 2001. The thermal images illuminated distinct areas of geothermal activity along the southern caldera wall. In combination with meteorological data the images were used to estimate surface temperatures and heat flux from patches of open water along the margin of the ice shelf covering the Grimsvotn subglacial lake. It was found that water temperatures varied from 0°C to ∼45°C and that the heat flux to the atmosphere from open water varied from slightly negative values (net energy gain) up to 1000 W m-2. The total heat output from the ∼0.1 km2 of open water was estimated as ∼18 MW, about 1% of the base heat output of Grimsvotn. The aerial photographs were used to produce geomorphological maps of the caldera wall, including areas that cannot be safely mapped from the ground. This work indicates that thermal imagery can be an important supplement to ground-based measurements, and that combined optical and thermal remote sensing is a useful tool for spatially detailed monitoring of inaccessible and partly ice-covered volcanoes.