The perturbed Lennard-Jones chain (PLJC) equation of state (EOS) is extended to polymer mixtures. This equation utilizes the hard-sphere chain mixture as the reference system, while the perturbation term is derived based on the first-order variational theory. Binary vapor-liquid equilibrium (VLE) data of a wide range of polymer mixtures are well described by this model through the use of one binary parameter k(ij). We also show that the PLJC model is able to predict liquid-liquid equilibrium of polymer-solvent systems satisfactorily by introducing an empirical parameter that adjusts the degree of perturbation in the model. All phase equilibrium calculations in this study were performed rigorously by global minimization of Gibbs free energy using a genetic algorithm.