A mixing rule for cubic equations of state (CEoS) applicable to all types of system asymmetries-referred to hereafter as the universal mixing rule (UMR)-is proposed. For the cohesion parameter of the CEoS, the mixing rule involves the Staverman-Guggenheim part of the combinatorial term and the residual term of the original UNIFAC Gibbs free energy expression. For the covolume parameter of the CEoS, the quadratic concentration-dependent mixing rule is used with the combining rule for the cross parameter b(ij) = [1/2(b(i)(1/2) + b(j)(1/2))](2). This UMR is applied to the volume-translated and modified version of the Peng-Robinson equation of state of Magoulas and Tassios (Fluid Phase Equillb. 1990, 56, 119), leading to what is referred to as the UMR-PR model. Very satisfactory results are obtained using the existing interaction parameters of the original UNIFAC model for vapor-liquid equilibrium predictions at low and high pressures for a wide range of system asymmetries including mixtures containing polymers. Satisfactory liquid-liquid equilibrium predictions are also obtained with the UMR-PR model.