The H-to-D vapour-pressure isotope effect in liquid ammonia has been measured at 62 temperatures between 228 K and 260 K. The vapour pressures, corrected to 100 per cent nuclidic purity, have been fitted to the equation: T In r = A + BIT + CT, where r is the vapour-pressure ratio p(NH3)/p(ND3). The fit yielded the parameters: A --8.22508 K, B = 12338.2 K 2, and C = -0.05544. Comparisons with the results of other authors were made in order to clarify some discrepancies found in the literature. Our values are in accord with the previous results of King et al, and an extrapolation of the fitted equation down to the triple-point temperature gave good agreement with the published results. The fitted equation was used in conjunction with the Clapeyron equation to calculate the difference in the molar enthalpies of vaporization between NH 3 and ND 3. At T = 230K that difference is -846 J. tool 1 decreasing to -747 J. mol 1 at ~260 K.