The vapour pressure difference between CHCl3 and CDCl3 is measured by differential capacitance manometry between -60 and +60 °C. The results can be expressed by the equation ln(pH /pD) = 5.507 × 10-3-6.269/T. The vapour pressure of CHCl3 between -98 and +62°C has also been determined. The experimental H/D vapour pressure isotope effect data together with the available literature values on the 12C/13C and 35C1/37Cl isotope effects are interpreted within the framework of the statistical mechanical theory of isotope effects in condensed systems. The results show that the anharmonicity constant of the CH stretching vibration decreases by about 4% on the vapour-liquid transition.