The rate of reaction of ground state O+ with H2O has been measured in the energy range, ECM, 0.032-11 eV. The measurements show that the rate coefficient does not decrease drastically with increasing energy, contrary to previous work. The rate constant measured here is then applied to a simple model, which includes reactions of O+ with water dimers, to calculate water concentration in the vicinity of the Space Shuttle. The results suggest that the pressure of water vapor may be lower than previously thought with an upper limit of 1¿109 molecules cm-3. ¿ American Geophysical Union 1989