This study concerns the morphology development of a blend based on a thermoplastic matrix with a reactive system undergoing polymerization. The system consists of 60 weight percent of polystyrene and 40 weight percent of a thermoset precursor with a low reactivity diamine (4,4-methylenebis(2,6-diethylaniline)). Experiments were carried out in a temperature controlled oil bath and in an internal mixer. In the first case, the reaction induced phase separation process was conducted under quiescent conditions and, in the second case, under dynamic conditions. Although the kinetic study showed no noticeable difference between the static and dynamic experiments, the final morphologies obtained are quite different depending on shear. Whereas spherical particles (similar to 3 mum) are obtained under static conditions, irregular particles and larger dimensions characterize the morphology under shear. The classical phenomenon of droplet break up/coalescence is not applicable when an insoluble fraction is present. In the vicinity of the gelation time, the particles tend to aglomerate in an irreversible way and so the process of shape relaxation leading to spherical particles becomes impossible. (C) 2004 Elsevier Ltd. All rights reserved.