The difference between the vapour pressures of HCI and DCI has been measured over the temperature range 17lj203 K by a differential manometric technique in a precision cryostat. In this range the vapour pressure of HCI is higher than that of DCI by 3.2% at 170 K, decreasing to 0.9% at 200 K. The reduced panition function ratios fi fs derived from the vapour pressure data can be described by the equation ln(fi/f,) = (3914.5ii l O) / p -(17.730*0.055)/T. The experimentally obselved H-D vapour pressure isotope effect, together with the values on the 35CI-3' C1 isotope effect available in the literature, is interpreted in the light of the statistical theoq of isotope eKecls in condensed systems by using spectroscopic data of the vapour and liquid phases. The m u l l s indicate that the rotation in liquid hydrogen chloride is hindered. Temperature-dependent force mnstanls for the hindered translational and rotational motions were invoked in order to obtain better agreement between the model calculation and experiment.