Molecular dynamics simulations of united atom Lennard-Jones model for methane, ethane, and propane in TIP3P water have been used to estimate the chemical potentials of aqueous methane, ethane, and propane from 600 to 1200 degrees C and densities from 0 to 1 g cm(-3). Errors in the simulation itself are negligible. Estimates of the errors in the predictions due to the inadequacy of the models show that this method of prediction gives reasonable accuracy. The calculations are not sensitive to the dipole moment of the water molecule or to the well depth (epsilon) of the water-methane interaction. The diameter (sigma) of the water-methane interaction is the most important parameter. The present predictions for methane are compared with a variety of other methods from the literature. An equation with seven adjustable parameters is presented which fits all of the simulation results as a function of temperature, density of water, and number of carbon atoms. This equation should allow reasonable extrapolations to predict the properties of butane, pentane, and hexane. An even simpler equation with only two adjustable parameters is able to fit all of the experimental data in this temperature and density region if the calculated solute-water second virial coefficient fur this model is used. The fit is not quite as accurate as with the seven-parameter equation, but this equation should be useful for predictions of normal and branched hydrocarbons, Equations of state can give good predictions which are more accurate than those of Shock et al. (Giochem. Cosmochim. Acta 1989, 53, 2157; 1990, 54, 915. J. Chem. Sec., Faraday Trans. 1992, 88, 803), but use of molecular dynamics simulations to predict the properties of aqueous nonelectrolytes at very high temperatures is the method of choice for making these predictions.