Publication costs assisted by Atomic Energy of Canada LimitedThe overall deuterium isotope separation factor between hydrogen and liquid water, a, has been measured directly for the first time between 280 and 370 K. The data are in good agreement with values of a calculated from literature data on the equilibrium constant for isotopic exchange between hydrogen and water vapor, Ki, and the liquid-vapor separation factor, • The temperature dependence of a over the range 273-473 K based upon these new experimental results and existing literature data is given by the equation In a = -0.2143 + (368.9/T) + (27 870/T2). Measurements on ay given in the literature have been surveyed and the results are summarized over the same temperature range by the equation In ay = 0.0592 -(80.3/T) + (25 490/T2).