Reactive melt blending of polyamide 6 (PA) and polysulfone (PSU) was carried out and the interfacial behavior of in situ-formed block copolymer was studied. A series of reactive PSUs with different functional groups at chain ends were prepared and used; PSU-epoxy, -triazine and -phthalic anhydride. The morphology development during melt blending was investigated by light scattering and transmission electron microscopy. The results suggested that when coupling reaction quickly proceeded and the in situ-formed copolymers were densely accumulated at the interface, they could be easily pulled out by external shear forces to form micelles in matrix. This micelle formation led to 10 nm order dispersion as a whole at late stages of mixing. By contrast, the pull out did not take place when the coupling reaction was slow and the copolymer chains were less accumulated at the interface. In this case, the in situ-formed block copolymer acted as a simple emulsifier to yield sub-mu m level dispersion. The difference between the fast and slow reaction systems was discussed in terms of a balance between thermodynamic stability of copolymer chains at the interface and hydrodynamic effect for the pull-out. (C) 2000 Elsevier Science Ltd. All rights reserved.