Abstract Nadir-looking microwave radiometers are flown on altimeter missions to correct the altimeter range for water vapor path delay. This paper describes a technique to intercalibrate the brightness temperatures of the ERS-1 and ERS-2 microwave radiometers by using the TOPEX radiometer as a common reference. The technique is based on the analysis of radiometer measurements made at crossover points between TOPEX, ERS-1, and ERS-2 orbits with less than 1-h time lags. This provides about 1000 comparison points within 4 months and covers most atmospheric and oceanic states. It is shown first that the ERS-2 radiometer brightness temperatures need to be corrected to make them consistent with ERS-1 and thus to ensure the homogeneity of the series of ERS altimeter data. The accuracy of the method is then estimated from the results of a 3-yr TOPEX/ERS-1 data comparison and is shown to be about 0.2 K for the ERS-2 23.8-GHz channel and about 0.4 K for the 36.5-GHz channel. This 3-yr comparison also shows a −1 mm ...