The preparation of a series of blends containing poly(butylene terephthalate) (PET) and nitrile rubbers of varying acrylonitrile contents is reported. The nitrile rubbers form a dispersed phase within the polyester matrix to give blends with enhanced fracture resistance. The tensile and fracture properties of the materials are related to the blend morphologies as determined by transmission electron microscopy. It is suggested that the effectiveness of nitrile rubbers in the toughening of PET may result from the relatively close match of the cohesive energies of the two components, or from the irregular shape of the rubber particles.