We used in situ reactive compatibilization to investigate the interfacial reinforcement between polypropylene (PP) and nylon 6 (Ny6). A certain amount of maleic anhydride grafted polypropylene (MAPP) was pre-blended with pure PP to form in situ a copolymer with Ny6. The fracture toughness was measured using an asymmetric double cantilever beam test (ADCB). An analysis of the locus of failure revealed that at constant bonding temperature, the fracture toughness between PP and Ny6 was influenced not only by the bonding temperature but also by the bonding time. The fracture toughness increased with the bonding temperature until 220 degreesC, and then decreased at higher bonding temperatures. which could be explained by the progressive occurrence of two different failure mechanisms, first adhesive failure at the interface and later cohesive failure between chains. X-ray diffraction measurements on specimens prepared at bonding temperatures of 210, 220, and 230 degreesC revealed no identifiably different crystalline PP phases. The fracture toughness increased with the annealing time, passed a peak, and then reached a plateau. The dependence of the fracture toughness on the bonding time could also be explained in terms of the two fracture mechanisms. X-ray photoelectron spectroscopy (XPS) results corroborated these explanations. (C) 2004 Elsevier Ltd. All rights reserved.