The effect of fiber reinforcement on the crystallization of poly(ethylene terephthalate) (PET) in blends of PET and polycarbonate (PC) is determined using differential scanning calorimetry (DSC). Isothermal crystallization rate of PET in PET/PC blends (80% and 60% PET by weight) is depressed as a result of Kevlar 49 and glass fiber reinforcement. In contrast, the crystallization rate of the same PET (no PC present) is enhanced by the Kevlar. The degree of crystallinity and melting temperature of PET in fiber-reinforced PET/PC is also depressed relative to that of unreinforced PET/PC and fiber-reinforced PET. These results show that there is a significant interaction between the effects of fiber reinforcement and of the PC component on the crystallization of PET in PET/PC composites. The PET/PC blends studied display two glass transition temperatures, characteristic of an immiscible blend. When glass fibers are used as the reinforcing phase, the T-g of PET is depressed. Optical microscopy observations are also consistent with an immiscible blend. When Kevlar fibers lie in a crystallizable (PET) domain, crystallites are densely nucleated on the fiber surface to induce transcrystallinity. Glass fibers produce less densely nucleated surface crystals in these systems.