Isothermalisobaric Monte Carlo molecular simulations are performed to estimate the vaporphase fugacity coefficients of hydrogen fluoride and water in their mixtures, covering the full range of composition at experimentally known saturation temperatures and 1 atm pressure. These results are combined with three sets of established experimental data for vapor-liquid coexistence in this system to compute corresponding liquid-phase activity coefficients for each. Application of an integral thermodynamic consistency test shows that the new values of the activity coefficients are greatly improved over previously reported values, which had been computed using an ideal-gas treatment for the vapor-phase. The consistency test also gives reason to prefer the data of Munter et al. [Ind. Eng. Chem. 1947, 39, 427] and Vieweg [Chem. Tech. (Berlin) 1963, 15 (12), 734] in relation to results reported more recently by Miki et al. [J. Electrochem. Soc. 1990, 137 (3), 787], but any such conclusions should be taken only with due consideration to the approximations inherent in the analysis.