Abstract: The equilibrium constants K g for the isotopic exchange reactions between hydrogen and water vapor are calculated using the ratios of partition functions of the molecules of reactants and products. Saturation vapor pressures of the isotopic waters are adopted to evaluate the separation factors α g-l for the isotopic exchange reactions between water vapor and liquid water. Eventually, the K g and the α g-l give the total separation factors α tol for the isotopic exchange reactions between hydrogen and liquid water. Here we present the calculation results of the HD-H 2O and DT-D 2O isotopic exchange reactions in the temperature range of 283.2～373.2 K at the pressure 0.1 MPa.