Measurements of the H2O emission line at 22.2 GHz show that the concentration in the mesosphere of water vapor moleucles can be highly variable, ranging from nearly constant mixing ratios of 2--4 ppmv, up to a layer near 65 km altitude with peak values around 12 ppmv. Results above 70 km altitude have been used to deduce a value for the vertical eddy diffusion coefficient of (4.4¿0.7)¿105 cm2 sec-1.