We present a photochemical model describing the changes in the isotopic composition of stratospheric water vapor that result from methane oxidation and reactions with O(1D). We then compare model calculations with measurements made with the Smithsonian Astrophysical Observatory far-infrared spectrometer during seven balloon flights that took place between 1989 and 1997. The agreement between the model calculations and measurements establishes the validity of the model and the internal consistency of the data set. Finally, we use the model and measurements together to estimate the mixing ratio and isotopic composition of water vapor entering the stratosphere and find that the average over all flights of the water vapor mixing ratio, ΔD, Δ18O, and Δ17O are 3.48¿0.15 parts per million by volume, -679¿20%, -128¿31%, and -84¿31%, respectively, where the errors indicate the estimated accuracy. ¿ 2001 American Geophysical Union <