A detailed analysis of the H2O 6.9 μm limb radiance measured by the improved stratospheric and mesospheric sounder on board the Upper Atmosphere Research Satellite has been conducted using a non-local thermodynamic equilibrium (non-LTE) radiative transfer model. There are important non-LTE effects in the measured radiance, both in the pressure modulator (sensitive only to the lines of the v2 fundamental band of H2O) and the wideband signal (similar to that of a conventional radiometer) above about 50--55 km. They show no variability with the solar zenith angle and a strong dependence on mesospheric temperatures. We have found that these effects are due to the departure from LTE of the (010) and (020) levels of H2O in the mesosphere. A value between 1.0 and 3.0¿10-12 cm3 s-1, close to the lower limit of the measurements, has been deduced for the rate coefficient of the vibrational exchange between the &ngr;2 vibrational mode of H2O and O2(1). Also, evidence of the excitation of O2(1), with a quantum yield between 2 and 6, after photolysis of O3 in the Hartley band has been found in the measurements. ¿ 1998 American Geophysical Union