This project culminated in the development of a new flow method and device for measuring the water contents of high pressure gases. This new flow method uses both an electrical resistance sensor and/or a chemical desiccant method to measure the water contents of methane, ethane and methane + carbon dioxide and ethane + carbon dioxide gas mixtures from 3.4 MPa to 110 MPa over a temperature range from 310 to 477 K. The resulting measurements have reduced uncertainty in the binary experimental results to between 2 to 7 percent, and give ternary results with an uncertainty between 5 and 14 percent. The new experimental data are modeled using a hybrid method which combines the Peng-Robinson equation of state to calculate vapor phase fugacity coefficients with a highly accurate equation of state to calculate the fugacity of water. Gas solubilities in the aqueous phase are calculated using a Henry's law coefficient, while aqueous fugacities are calculated using the NIST/ASME equation of state for water, while the effects of salts are incorporated using Pitzer correlations for the activity of brines. The new model is able to predict ternary phase equilibria using interaction parameters fit to binary data. This allows predictive phase behavior calculations to be made for multiple components. The related system methanol + alkanes, where the alkane was a member of the homologous series propane to decane was modeled using the Statistical Associating Fluid Theory (SAFT) equation of state for both liquid-vapor (VLE) and liquid-liquid equilibria (LLE). It was shown that the PC and CK-SAFT equations of state were capable of representing the phase behavior to within a few percent (generally 1--4%) of the experimental data using binary interaction parameters that were weak linear functions of temperature and alkane molecular weight. The binary interaction parameters were fit to the VLE data and then applied to the LLE data with excellent results for the methanol + alkane systems from 1 to 150 MPa. For alkanes longer than octane, systematic deviation was observed.