Interfacial tension measurements of polyamide/polypropylene (PA6/PP) interfaces are reported at high temperature, using a spinning drop tensiometer, especially adapted to the study of the effects of copolymers. Copolymers in different amounts are included in the PP drop, and their migration towards the interface is inferred from the evolution of the drop diameter during the experiment. The importance of the compatibilizer is studied; small amounts of copolymer give rise to a significant decrease of the interfacial tension. This is due to effective migration of the copolymer, as deduced from diffusion coefficients. Above a critical concentration, the interfacial tension increases again. This phenomenon has not been observed before for such systems and is attributed to the presence of micelles in the bulk which prevent the copolymer migration through the creation of yield stresses.