The equilibrium fractionation factors of oxygen and hydrogen isotopes between liquid water and water vapor have been precisely determined from 25 to 350°C on the VSMOW-SLAP scale, using three different types of apparatus with static or dynamic techniques for the sampling of water vapor. Our results for both oxygen and hydrogen isotope fractionation factors between 25 and 100°C are in excellent agreement with the literature (e.g., Majoube, 1971). Our results for the hydrogen isotope fractionation factor above 100°C also agree quantitatively with the literature values of Merlivat et al. (1963) and Bottinga (1968). The results for the hydrogen isotope fractionation factor obtained in this study and from most of the literature were regressed to the equation, 103Inα1-v(D) = 1158.8( T3 109) -1620.1 ( T2 106) + 794.84( T 103) -161.04 + 2.9992( 109 T3), from 0°C to the critical temperature of water (374.1°C) within ± 1.2(1σ) (n = 157); T(K). The cross- over temperature is 229 ± 13°C (1σ). Our values for the oxygen isotope fractionation factor between liquid water and water vapor are, however, at notable variance with the only dataset available above 100°C in the literature (Bottinga, 1968), which is systematically higher (av. + 0.15 in 103 In α1-v(18O)) with large errors (± 0.23 in 1σ). Our results and most of the literature data below 100°C were regressed to the equation, 103 In α1-v(18O) = -7.685 + 6.7123( 103 T) - 1.6664( 106 T2) + 0.35041 ( 109 T3), from 0 to 374.1°C within ± 0.11 (1σ)(n = 112); T(K). A third water-steam isotope geothermometer, using the ratio of ΔδD/Δδ18O given by water and steam samples, is readily obtained from the above equations. This geothermometer is less affected by incomplete separation of water and steam, and partial condensation of steam than those employing the oxygen and hydrogen isotopic compositions alone.