Abstract We report measurements of the equilibrium D H fractionation factor (a) between methane and hydrogen in the temperature range 200–500°C. Isotopic equilibrium was achieved by recycling the gases over a Ni-Thoria catalyst, using an in-line sampling volume for sequestering aliquots of the gas mixture without contributions from adsorbed gases on the catalyst. Equilibrium values of a were approached from both sides by use of (1) enriched CH 3D in the initial mixture and (2) pre-equilibration of the gases at temperatures below that of the final equilibrium mixture. The measured values of a are linear vs. 1 T 2 and fit the equation a = 0.8994 + 183,540/T2, with a standard deviation σ = ±2.5‰. The D H fractionation factors for water vapor-hydrogen exchange measured by Suess (1949) and by Cerrai et al. (1954) are also linear in α vs. 1 T 2 over the temperature range of the data: comparison with published D H ratios in high-temperature (1127°C) volcanic gases at Surtsey volcano shows that the Suess (1949) data are much closer to the observed ratios in H2 and H2O. The Suess (1949) measurements (80– 200°C) are also much closer to the theoretical values calculated by Bardo and Wolfsberg (1976), which fit the observed Surtsey fractionations slightly better than the extrapolated Suess (1949) results. We conclude that (1) the Suess (1949) measurements are the better set of experimental data, (2) the Surtsey gases are close to isotopic equilibrium at the vent temperatures, and (3) the Bardo and Wolfsberg (1976) theoretical equation gives the best representation of the H 2OH 2 fractionation factors. This equation is combined with the Horita and Wesolowski (1994) equation for H 2O liquid-vapor fractionation factors and can be used with the CH 4HZ a values to determine whether concordant temperatures are observed in the system CH 4H 2H 20. Application to the D H ratios in the East Pacific Rise hydrothermal vents measured by Welhan and Craig (1979) shows that concordant temperatures are obtained for both CH 4H 2 and H 2OH 2 data, and are close to the approximate vent temperatures (∼350°C). We note that fractionation equations in which a, rather than In a, is fit to powers of T are much more useful for geochemical studies because the precision estimate is uniform over the entire temperature range of the data.