The weakness of plastics at weld lines provides serious difficulties far the design and long term durability of injection molded parts. The goal of this work was to identify the cause of weld line weakness in polypropylene (PP) systems. The morphology of weld lines in a high molecular weight PP has been studied. It was found that the PP contains a hindered phenolic antioxidant additive that is not soluble in the polymer at the standard processing conditions. Transmission electron microscopy (TEM) pictures reveal the additive existing as a dispersed phase in the bulk polymer. Even though very small concentrations of this additive are normally used, (0.1-0.5%) large quantities were found at weld lines in a band approximately 100 nm wide and penetrating about 10 mu m into the surface of the part, hindering strength development at the weld line. X-ray photoelectron spectroscopy (XPS) results confirm enhanced concentrations of antioxidant on the now front and mold wall surface of short shot samples. The mechanical properties (Izod impact, tensile strength) are measured for samples molded at various processing conditions, varying amounts of antioxidant additive and with and without weld lines. The results are consistent with the presence of the additive playing a key role in strength development at PP weld lines.