Bond formation between maleic anhydride-g-polypropylene (PPg) and polyamide 6 (PA) by in situ block copolymer formation has been investigated. The effects of bonding temperature and time on the critical strain energy release rate, G(C), of the bonds were studied. The G(C) values were measured using a wedge test in an asymmetric double cantilever beam geometry. Electron spectroscopy for chemical analysis (ESCA) and scanning electron microscopy (SEM) observations of the fracture surfaces were used to provide detailed information on the locus of failure and the failure mechanisms. An increase of G(C) with bonding temperature was observed with two well defined transitions corresponding to the melting temperature of PPg and PA. Below the PPg melting temperature, there is no significant adhesion due to the absence of intimate contact between the adherends. Above this temperature, G(C) increases gradually with temperature. This is explained by the increased mobility of maleic anhydride grafted PP chains which can migrate towards the interface and react with the amine end-groups of the PA. Optimal bonding, however, requires melting of both polymers and results in the highest G(C) values which approach the cohesive G(C) of PPg. Analysis of these fracture surfaces via ESCA and SEM have shown that failure was cohesive in the PPg and accompanied by significant plastic deformation. This is interpreted as saturation of the interface by block copolymer due to the mutual migration of the PA and grafted PP polymer chains that becomes possible when the PA melts. It is suggested that as crystallization proceeds each half of the block copolymer is incorporated into crystalline domains on their respective sides of the interface, producing the highest G(C) values observed.