The compatibilization effect of poly(styrene-b-methyl methacrylate) on the immiscible polymer blend of polystyrene and poly(styrene-co-acrylonitrile) was examined by dynamic mechanical spectroscopy. Special regard was taken of additional relaxation processes in the flow region, which were analysed using weighted relaxation-time spectra. Furthermore, an important topic of our research was the characterization of the morphology by energy-specific transmission electron microscopy. By increasing the amount of compatibilizer, the average particle size decreases from large polystyrene domains (R similar or equal to O.2 mu m) down to micellar dimensions (R similar or equal to 60 nm). In this way, typical morphological structures in the blends were related with theological behaviour. With a modified emulsion model from Choi and Schowalter, some correlations between particle size and trends in interfacial tension could be established dependent on compatibilizer concentration. Especially, it was found that the particle size reduction and the reduction of interfacial tension occur in different ranges of block-copolymer concentration.