The toughness and morphology of poly(butylene terephthalate), PET, blends with acrylonitrile-butadiene-styrene, ABS, materials are very sensitive to the PET molecular weight or melt viscosity. High PET melt viscosity is shown to lead to improved ABS (and styrene acrylonitrile copolymer, SAN) dispersion which improves the low temperature fracture toughness of PBT/ABS blends. Compatibilization by methyl methacrylate, glycidyl methacrylate, ethyl acrylate terpolymers, or MGE, was shown to further improve ABS and SAN dispersion and reduce the ductile-brittle transition temperature. Addition of the compatibilizer broadens the window of acceptable processing temperatures. The minimum quantity of ABS required to toughen PET is lower the higher the PET melt viscosity. The tensile properties of PBT/ ABS/MGE blends were shown to be relatively insensitive to PET melt viscosity or reactive compatibilization.