The phase diagram of transition temperatures versus the mole fraction x of the liquid-crystalline component PHB was determined for a series of copolymers PET/xPHB, where PET = poly(ethylene terephthalate) and PHB = p-hydroxybenzoic acid. The diagram includes both equilibrium and nonequilibrium phases and is based on results reported here as well as on those of earlier investigators and on results obtained by several techniques, with identical samples studied by the same techniques but at different locations. The diagram is fairly complex. The quasi-liquid phase reported earlier is discussed in some detail. The importance of the diagram for intelligent processing is discussed.