Morphology of reactively compatibilzed polymer blends was discussed in terms of particle asphericity (xi)-size (a) map. Reactive melt blending of polysulfone (PSU) with polyamide 6 (PA) was carried out at 20/80 (PSU-PA) wt ratio by adding a few wt% of reactive PSUs; maleic anhydride-grafted PSU (PSU-MAH) or phthalic anhydride-terminated PSU (PSU-PhAH). Morphology development was investigated by light scattering, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM picture was analyzed by a new digital image analysis (DIA), based on the effective ellipse method. The new DIA allows us to analyze the particle size, the shape (asphericity) and their distributions, which are reminiscent of size reduction process and shape relaxation during melt blending. In in situ graft copolymer-forming (PSU-MAH) systems, the xi-a map shifted rather vertically to higher asphericity and smaller particle sides as the loading amount of reactive PSU increased, suggesting an emulsifying effect of the in situ-formed graft copolymer. By contrast, in in situ block copolymer-forming (PSU-PhAH) systems, the xi-a map shifted rather horizontally to the smaller particle-side with an increase in the loading amount of reactive PSU. The difference in the xi-a map between the in situ graft copolymer- and block copolymer-forming systems was interpreted by the "pull-out or not" of the in situ-formed copolymers from the interfacial region by shear forces during mixing. The pullout of in situ-formed block copolymer was confirmed by TEM: tiny domains (20-40 nm in diameter) coexisted with the bigger particles (mu m) with low xi (probably, "naked particles"-not or less covered with the copolymers). (C) 2000 Elsevier Science Ltd. All rights reserved.