Transmission electron microscopy has been used in conjunction with RuO4 staining to investigate the structure of (iPP/iPP-g-MA)-PA6 reaction bonded interfaces and microdeformation at interfaces which had failed by crack propagation along the interface. In relatively weak interfaces, the crack tip was preceded by a single, well-defined crazelike fibrillar deformation zone on the iPP/iPP-g-MA side of the interface, with fibril diameters of between 10 and 15 nm. The crack tip itself also propagated along the interface, providing some justification for the extension of established models for interfacial failure in glassy polymers to the present case of two semicrystalline polymers. In stronger interfaces, however, considerable diffuse deformation was observed to accompany crack propagation. This was accounted for by the deeper penetration of the damage zone into the iPP/iPP-g-MA, into regions where the microstructure was less well ordered than immediately adjacent to the interface. This transition from a well-defined to a diffuse damage zone may modify the relationship between the local interfacial strength and the global fracture toughness.