Monte Carlo simulations were used to calculate water-methane and water-ethane phase equilibria over a wide range of temperatures and pressures. Simulations were performed from room temperature up to near the critical temperature of water and from subatmospheric pressure to 3000 bar. The Henry＇s law constants of the hydrocarbons in water were calculated from Widom test particle insertions. The Gibbs ensemble Monte Carlo method was used for simulation of the water-rich and hydrocarbon-rich phases at higher pressures. Two recently proposed pairwise additive intermolecular potentials that describe : accurately the pure component phase equilibria were used in the calculations. Equations of state for associating fluids were also used to predict the phase behavior. In all cases, calculations were compared with experimental data. For the highly nonideal hydrogen bonding mixtures studied here, molecular simulation-based predictions of the mutual solubilities are accurate within a factor of 2, which is comparable with the accuracy of the best equations of state.