. Can. J. Chem. 54, 3007 (1976). Precision isotope ratio mass spectrometry has been used to determine the relative vapour pressures of the four principal isotopic species of liquid carbon disulfide. Relative to the principal species '2C32S3'S = 1.000000. Introduction For a long time it was believed that substitution of a heavier isotope for a lighter isotope in pure polyatomic molecular liquids invariably decreased the vapour pressure of the liquid. This relation between the vapour pressure and molecular weight is usually described as a normal vapour pressure isotope effect (1). While many molecular systems do indeed display normal vapour pressure isotope effects ( H 2 0 and D 2 0 provide perhaps the best known example), other systems are known which show an inverse isotope effect, i.e., the heavier species has the higher vapour pressure. For example, it is well established that the vapour pressure of 13CH30H is greater than that of 12CH301-I (2). For some systems, both kinds of effects have been observed for a given element, with the isotope effect changing sign when the temperature is changed. The deutero-rnethanes are an example of this class (3). Perhaps even more unexpectedly, systems are also known for which an inverse isotope effect is produced by substitution in one part of a polyatomic molecule, and a normal isotope effect by substitution in another part of the same molecule (2, 4, 5).