Vertical profiles of H2O and CH4 were measured in situ in the Arctic winter during the European Arctic Stratospheric Ozone Experiment (EASOE) 1991/1992. The mixing ratio of 2⋅CH4+H2O was constant in the altitude region between the hygropause and 30 km without a significant difference for measurements made inside and outside of the polar vortex. During winter, the partitioning between the hydrogen compounds changes inside the polar vortex. In late November, typical prewinter conditions exist with H2O mixing ratios of 5 ppmv at the 550 K level. By late winter, mixing ratios for the same potential temperature have increased to 6 ppmv inside the vortex. Owing to the increase of the water vapor mixing ratio, the minimum temperature at which either nitric acid trihydrate or ice particles are stable will be approximately 1 K higher at the end of the polar winter inside the vortex when compared to prevortex conditions. ¿ American Geophysical Union 1996