A model for calculating second and third virial coefficients, originally developed to described the behavior of polar halocarbon refrigerants and their mixtures, has been applied to systems of natural gas hydrocarbons and their common impurities, namely H2S, H2O, CO2, CO, and N-2. It can be used to correlate data, point out probable errors and provide predictions where data are nonexistent or unreliable. It also provides a means for calculating accurate gas-phase densities, thermodynamic properties, fugacities, and "calculated data" which can be incorporated into global equations of state for phase equilibrium calculations. Examples are shown for some of the pure components and for the binary systems C2H6 + H2S and CH4 + H2O. In the case of H2O, terms are included that account for association. Comparisons are made with available data, and predicted values are shown where no data exist.