A method has been developed for measuring small differences between the vapour pressures of two samples. V.p. differences were determined between water samples with a high concentration of oxygen-18 (H2O18 and D2O18) and samples with a low concentration in the temperature range 15–90°C. Differences between the vapour pressures of water containing ∼55 % O17 and samples containing high concentrations of O16 and O18 respectively, in the range 40–90°C, have also been measured. Using these results, vapour pressure ratios between the pure isotopic species were calculated.