We present experimental results that show that the kinetic isotopic fractionation during gas exchange is 0.9972¿0.0002 for oxygen, 0.9992¿0.0002 for methane, 0.9987¿0.0001 for nitrogen, and 0.982¿0.002 for hydrogen, and that the equilibrium fractionation between water and gas phases is 1.037 for hydrogen. We show that the isotopic fractionation during gas transfer for these gases is not equal to the square root of their reduced mass in water, as would be predicted by an extension of the kinetic theory of ideal gases to dissolved gases. The use of isotopes as tracers of biogeochemical gases requires knowledge of the fractionation factor for air-water gas transfer; there have been few direct measurements of these factors. ¿ American Geophysical Union 1992