This paper presents a new method for the prediction of phase behavior in reservoir fluids using a four-parameter cubic equation of state. The parameters in the employed C7+-characterization procedure are calculated directly from measured molecular weight and specific gravity data for the hydrocarbon fractions. It is shown that PVT properties (e.g., liquid dropout) and phase equilibria can be accurately predicted even in the near-critical region by using the new method. Saturation points for a number of fluids containing significant amounts of non-hydrocarbons (CO2 or N2) are calculated. The predicted saturation points agree well with experimental data. Finally, it is shown that the new model is able to accurately predict liquid volumes of mixtures including those with a considerable content of N2.