A comparison is presented between sea surface temperature measurements from the Along Track Scanning Radiometer (ATSR) space instruments and coincident in situ measurements made using the high-accuracy Scanning Infrared Sea Surface Temperature Radiometer (SISTeR). The in situ measurements were obtained as part of the Mutsu Bay Experiment (MUBEX) conducted in northern Japan during July and August 1996. Differences between the satellite and in situ measurements are lower than 0.2 deg K for ATSR-2 and 0.5 deg K for ATSR-1. This indicates that both ATSR-1 (after 5 years of continuous operation) and ATSR-2 are operating well within their accuracy design specification.