Using a new Lyman α hygrometer, balloon-borne measurements of H2O were performed on September 20, 1993 at midlatitudes (43 ¿N). A cryogenic whole air sampler to measure CH4 and H2 amongst other long-lived trace gases was operated on the same payload. The profile of the water vapor mixing ratio showed a well-pronounced hygropause of 4 ppmv at &thgr;=420 K increasing to 6.0 ppmv at &thgr;=920 K. For &thgr;>400 K, total hydrogen &Sgr;H2=H2O+2⋅CH4+H2 appeared to be constant at a mean value of 7.72¿0.13 ppmv; the overall accuracy of this value is 0.65 ppmv. From the correlation functions of CH4 with H2 and H2O the water vapor yield from methane oxidation is determined with high precision to be 1.975¿0.030. The globally averaged water vapor mixing ratio entering the stratosphere from the troposphere was found to be 3.9¿1.0 ppmv. Intercomparison with H2O and CH4 profiles obtained nearby from the Halogen Occultation Experiment (HALOE) on board the UARS satellite showed good agreement within the experimental errors. ¿ 1999 American Geophysical Union