The spectral region from 6.2 to 6.6 μm contains the strongest infrared bands of water vapor and is often used by satellite remote sounding instruments to obtain the middle atmosphere concentration of this gas. However, the diurnal variation in mesospheric H2O emission is not well understood owing to nonlocal thermodynamic equilibrium (non-LTE) processes. This has led to H2O retrieval problems with past satellite instruments. Spectral measurements taken by the Space Shuttle cryogenic infrared spectrometers and telescopes for the atmosphere (CRISTA 1) instrument have been used to study these infrared limb emissions. The H2O radiance shows a daytime enhancement in the mesosphere due to nonthermal collisions and solar absorption. We use a combination of measurements and modeling to determine consistent values for the collisional rate constants and photolysis quantum yield that affect the non-LTE processes. Our new estimates of these quantities should improve considerably the confidence in the mesospheric H2O retrievals that will be made by the EOS TIMED sounding of the atmosphere using broadband emission radiometry (SABER) and Aura high-resolution dynamics limb sounder (HIRDLS) instruments. ¿ 2000 American Geophysical Union