The effects of the processing parameters on the weld-line mechanical properties of polystyrene (PS) and polycarbonate (PC) were investigated. PS was very sensitive to the presence of a weld line, showing property reductions of up to 70%. However, this sensitivity was mainly connected to the surface notch at the weld line. When this notch was removed, behavior close to that of unwelded specimens was obtained. The injection temperature was the main processing parameter because it affected the macromolecular diffusion speed and, therefore, influenced the weld quality. A direct relationship between the distance of molecular diffusion and the fracture mechanism was established. PC had a low weld-line sensitivity, despite being an amorphous polymer like PS. The difference between these materials was connected to the different sizes of the surface defects and to the different entanglement densities, which influenced the relaxation time and the global behavior (brittle-ductile). (C) 2004 Wiley Periodicals, Inc.