Different measurements have shown that large water vapor mixing ratios occur in the upper mesosphere in high-latitude summer just under conditions of strongest solar insolation where photolysis should effectively reduce its value. The analysis of this finding by means of a high-resolution one-dimensional model supplies evidence that water vapor is autocatalytically formed below a crossover height of about 65 km and destroyed by photolysis above this altitude. We found three catalytic cycles of water vapor formation from the molecular hydrogen reservoir. Hydrogen radicals, resulting for the most part from water vapor by photolysis and oxidation by O(1D), act as catalysts. A great number of the hydrogen radicals return to water vapor, particularly under the condition of small atomic hydrogen concentration decreasing strongly with decreasing height. In the upper domain under the condition of large atomic hydrogen concentrations the formation of molecular hydrogen will be favored. Favorable conditions for large water vapor mixing ratios at high altitudes are given by an accelerated upward vertical wind. Below the crossover level the vertical wind should be weak so that H2 and CH4 have sufficient time to oxidize into H2O, a very slow process. Above this level, however, the vertical wind should be strong in order to lift the air, enriched with H2O, as quickly as possible to greater heights before the dissociation has an effective impact on the H2O distribution. Such accelerated upward vertical winds are typical of the mesosphere under certain seasonally latitudinal conditions. Calculations by means of our three-dimensional model COMMA-IAP yield seasonal-latitudinal patterns and variations approximately comparable with those of the HALOE observations. In the paper we discuss the chemical background and the interplay between dynamics and chemistry in more detail.