A comparison has been made between the Sanchez-Lacombe equation-of-state theory and the theory of Patterson for describing the phase behavior of various polymer blends. Phase diagrams have been determined as well as PVT data, in order to determine the reduction parameters of the polymers used. With these data the phase diagrams were simulated applying these two theories. Furthermore, the cause of compatibility or incompatibility, i.e., the influence of free volume and enthalpic effects were investigated. Both theories show surprisingly similar behavior with respect to the mentioned properties.