A simple and thermodynamically consistent method is presented to establish an equation of state for mixtures by using activity coefficient model parameters. All current solution models such as NRTL, van Laar, UNIFAC, or any other thermodynamic model can be used. The main feature of the method presented is that only a single scaling factor value determined at a given reference temperature is required to predict the vapor-liquid equilibria in a wide range of temperature and pressure. The performance of the method is tested on the prediction of the vapor-liquid equilibria at low, moderate, and high pressures for six binary systems (methanol-benzene, acetone- water, methanol-acetone, methanol-water, ethanol-water, and 2-propanol-water) and a ternary system (acetone-water-methanol). For comparison, vapor-liquid equilibrium calculations were carried out with the Wong and Sandler method by using the PRSV equation of state associated with the van Laar and scaling factors. On the whole, it is found that at high pressures both methods give similar predictions but at low pressures the proposed method gives sometimes better results than that of Wong and Sandler method.