Radiative transfer calculations show that measurements in two relatively narrow spectral domains, corresponding to the atmospheric infrared window and to a band of water vapor absorption, can account for nearly all of the variance of outgoing longwave radiation integrated over the entire thermal spectrum. Statistical analyses of simultaneous colocated Meteosat radiance and Earth Radiation Budget Experiment (ERBE) radiant exitance data, for the months of November 1984 and April and July 1985, yield regression coefficients for estimating longwave radiant exitance MLW from radiances observed in the Meteosat infrared (IR) window and water vapor (WV) channels, with root-mean-square errors of the order of or less than 10 W m-2. The coefficients thus obtained are in good agreement with coefficients relating MLW, IR and WV from analysis of results of radiative transfer calculations. Meteosat data can then be used to estimate MLW at times not sampled by ERBE. Although some biases appear to exist over areas of heterogeneous cloud cover, they should not significantly affect studies of strong diurnal variations. ¿American Geophysical Union 1991