One of the most provocative recent developments in mesospheric science is the discovery of a narrow layer of water vapor located near 70 km altitude where the H2O mixing ratio reaches the highest values observed in the middle atmosphere [Summers et al., 1997a>. This layer can only be explained by a local source of H2O, which is contrary to conventional gas phase chemistry that predicts net H2O destruction in that part of the mesosphere. Although suggestions have been made that this layer is produced by an influx of small comets releasing water in the upper atmosphere, we propose that the reaction O+H2→H2O on the surface of meteoric dust can account for the observed H2O layer. Using a chemical-transport model we find that the inclusion of this reaction with a reaction probability of ~0.01 yields a model H2O layer with the observed characteristics. We suggest that the key to understanding the origin of the H2O layer lies in coincident observations of mesospheric H2 and H2O.