The use of high-intensity ultrasound for forming copolymers by sonication of mixtures of homopolymers is discussed. It is shown that the block copolymers formed in situ by this method can be used to compatibilize blends of immiscible polymers. The mixtures used in this preliminary study were polystyrene with either poly(cis-butadiene) or poly(methyl phenyl silane). The sonochemical method was found to be convenient and easy to use compared with the usual procedure of adding preformed block copolymers as compatibilizing agents.