In this research, a prediction method for the vapor-liquid equilibrium compositions of the propylene-propane mixture by the combined use of the COSMO-RS method and the Peng-Robinson equation of state is examined. Employing the van der Waals (VdW) mixing rule, the average deviations from this method for both the pressures and vapor phase compositions are no more than 0.62% and 0.75%, respectively. The average deviations for the relative volatility and tray efficiency are 0.93% and 5.52%, respectively. This method successfully describes the relative volatility of this system is not only related to the temperature but also dependent on the propylene mole fraction at low temperatures and high propylene concentrations. The density, excess molar volume and surface tension data are listed. The calculations explain the irregular phenomenon of the relative volatility. The results show that the non-ideality of this system comes from the dispersions between the components dominates the bulk liquid properties at low temperatures and high propylene concentrations. (C) 2018 Elsevier B.V. All rights reserved.