AbstractThe vapour‐liquid separation factors of the isotopic systems H 216O/H 218O and D 216O/D 218O have been determined by means of Rayleigh‐distillation of H2O and D2O respectively. The results obtained by mass‐spectrometric evaluation of the isotopic composition of oxygen in the water samples show that the separation‐parameter (δ = ln α) of the system D 216O/D 218O is some 10–15% lower than that of the system H 216O/H 218O. The temperature dependence of the separation‐parameters of both systems is given, showing generally poor agreement with the results of other authors concerning the system H 216O/H 218O. This investigation leads to a general principle stating that in compounds containing two species of isotopic elements, the isotope effect of the first element upon the separation‐factor of the isotopes of the second element has the same magnitude as the isotope effect of the second element upon the separation‐factor