A THEORY of the vapour pressure relationship for isotopic molecules has been given by Topley and Eyring1. It is, however, difficult to see a priori how to apply the theory in calculating vapour pressures for a particular case, in view of our lack of knowledge of the liquid state. The isotopic molecules of carbon tetrachloride would be expected to be very favourable cases for showing differences of vapour pressures if they exist. Grimm2 carried out a multiple fractionation with a single small packed column and calculated that neighbouring isotopic molecules on the mass scale had boiling points not differing by more than 0.001° C. We have calculated from Grimm's results that his total fractionation was equivalent to 75 theoretical plates.