Atmospheric hydrogen, H2, is removed to a large extent by reaction with OH. That reaction is expected to show a large kinetic isotope effect, enriching the deuterated hydrogen molecule, HD, in the atmospheric H2. We measured the kinetic fractionation factor using a static system with a UV- irradiated mixture of O3, H2O, and O2 to produce OH, H2 with a natural D content, and standard isotope mass spectrometry to measure the D/H ratio in the hydrogen. At room temperature the fractionation factor was found to be α=1.65¿0.05 (¿1&sgr;), where a is the ratio between the rate constants of the H2 and HD reactions with OH. A calculation based on BEBOVIB-IV gave exactly the same result and was extended to include the corresponding kinetic fractionation factors of other isotopic H2 molecules and their temperature dependence. The possible applications of the obtained fractionation factors to the tropospheric HD and HT balance and the resulting information for the H2 cycle are briefly illustrated. ¿ American Geophysical Union 1989