Grímsvötn, Iceland's most active volcano, is also one of the most powerful geothermal areas in Iceland. This subglacial volcano is located in the centre of Vatnajökull, Europe's largest temperate ice cap, and it erupted most recently in 1998 and 2004. As part of continuing research on heat flux, morphological changes and volcanic processes at Grímsvötn, thermal anomalies were mapped using remote sensing Natural Environmental Research Council (NERC) Airborne Research and Survey Facility (ARSF) data. The 2001 Airborne Thematic Mapper (ATM) thermal images of the Grímsvötn subglacial caldera reveal distinct areas of geothermal activity and provide an overview of the thermal anomalies associated with water and rock exposures. A crater lake located on the 1998 eruption site is shown to have a surface temperature of 30–35 °C. There is a good correlation between the ARSF data and ground-based temperature measurements. The thermal images also revealed previously undetected areas of high heat flow. Factors that complicate the interpretation and comparison of different datasets from an ice-covered area include recent cornice collapses and variations in atmospheric humidity. To reduce uncertainty in future missions, temperature measurements should be made at points whose position is well constrained using differential global positioning system. In addition, humidity and temperature measurements should be made at the time of flight.