Assemblies of polyamide 6 (PA6) with both syndiotactic and isotactic polypropylene (sPP and iPP) have been investigated in the presence of sPP-PA6 diblock copolymers formed in situ at the interface during annealing. Both iPP/PA6 and sPP/PA6 interfaces were stabilized by the sPP-PA6 copolymers. Crystalline orientations at the interface, characterized by X-ray diffraction using thin film assemblies, demonstrated an epitaxial crystallization of sPP on PA6, with a degree of orientation of the sPP increasing with increasing cooling rates, annealing times, and temperatures. For iPP/PA6 samples, the sPP-PA6 diblock copolymer had a sufficient stabilizing role on the interface to allow the observation of the iPP/PA6 epitaxy previously described, with a higher sensitivity of the degree of orientation to the annealing time than for sPP/PA6. The adhesive strength of macroscopic assemblies was also measured. The sPP-PA6 copolymer was able to mechanically reinforce the sPP/PA6 interface but not the iPP/PA6 one. The specific orientation of the polymers in the immediate vicinity of the interface is thus not sufficient to promote strong adhesion. The structure of the copolymer and its ability to entangle and probably cocrystallize with the matrix must also be considered.