Data for the fractionation of D/H and O-18/O-16 in water liquid-vapor equilibrium may be satisfactorily accounted for over the entire temperature range of coexistence with a rigorous thermodynamic relation and the assumption of ideal mixing of the binary isotopic water mixtures. D/H fractionation is used to clarify why the asymptotic behavior observed for many systems extends over a wide range of temperatures, but is very narrow for isotopic binary mixtures.