The pressure dependence of deuterium-hydrogen (D-H) fractionation in water to 500 degrees C and 200 megapascals has been calculated from high-temperature, high-pressure spectroscopic data, Pressure effects have a maximum at the critical temperature of water (20 per mil between 22 and 200 megapascals). Even larger effects are predicted for vaporlike densities from molecular dynamics simulations and molecular orbital calculations, Pressure effects explain many of the large discrepancies in published mineral-water D-H fractionation curves, Possible applications to natural examples include mineral-water isotope geobarometry.