The Meteosat geostationary satellites carry a filtered radiometer channel centered at 6.2 μm for the measurement of upper tropospheric humidity. The operational calibration is derived from radiative transfer calculations applied to radiosonde measurements; large fluctuations in the calibration have been noticed. Here, we apply another method based on collocations with NOAA/High-Resolution Infrared Radiometer Sounder (HIRS) channel 12. Radiative transfer calculations show that Meteosat measurements can be accurately predicted from HIRS channel 12 brightness temperatures. In addition, the HIRS instrument has onboard calibration of the complete optics, which makes it suitable as a reference for the calibration of other spaceborne radiometers. Application of the method to 3 years of Meteosat-5 data shows calibration fluctuations significantly smaller than those provided by the operational method. This result indicates that the short-term stability of the instrument calibration is much better than 5%, the variability given by the operational calibration. The proposed calibration coefficient is smaller by about 10--15%, which results in equivalent brightness temperatures biased by about 3--4 K and 35--50% relative error on the upper tropospheric humidity. Hypotheses of the causes for such large differences between the two calibration procedure results are proposed. ¿ 2000 American Geophysical Union