This work is aimed at an evaluation of the capability to predict the mechanical behaviour of blends on the basis of the phase diagram. Connections between phase structures, the phase diagram and mechanical properties were studied for binary blends of polycarbonate (PC) based on bisphenol-A with PET/0.6PHB, where PET = poly(ethylene terephthalate), PHB = p-hydroxybenzoic acid and 0.6 = the mole fraction of PHB in the copolymer. PET/0.6PHB is a longitudinal polymer liquid crystal (PLC) with LC sequences in the backbone along the main-chain direction. The techniques used were differential scanning calorimetry (d.s.c.), thermomechanical analysis (t.m.a.) in the penetration and three-point-bending modes, thermally stimulated depolarization (t.s.d.) and two kinds of dynamic mechanical analysis (d.m.a.) as a function of frequency omega and temperature T. The resulting phase diagram is plotted as a quasi-binary one (T versus concentration of PET/0.6PHB or PC) but provides information on miscibilities of all three pairs of constituents; it contains equilibrium as well as long-living non-equilibrium phases, including the quasiliquid. D.s.c. and d.m.a. results show that glass transition temperatures for the PC-rich phase drop by similar to 50 degrees C after 1 h of annealing, with evident consequences for mechanical properties. Variations of storage modulus and strain as a function of time are reported for different blend compositions. Isothermal storage moduli and results of three-point bending as a function of composition are connected to the phase diagram. Prediction of mechanical behaviour is possible from miscibilities of constituents in specific regions of the diagram.