Mechanical degradation and mechanochemical reaction in heterogeneous and homogeneous systems of poly(vinyl chloride) and poly(acrylonitrile-co-butadiene) polymer have been studied by ultrasonic irradiation at 30 degreesC. The rates of decrease in the number-average molecular weights of the degraded poly(vinyl chloride) and poly(acrylonitrile-co-butadiene) polymer in the swelled poly(vinyl chloride)-poly(acrylonitrile-co-butadiene) polymer solution were much faster than the homogeneous solution system and the final average chain lengths led to the smaller values than those in the latter system. On the other hand, mechanochemical reaction occurred by polymer radicals produced from the chain scissions of both polymers by ultrasonic irradiation. The changes in the composition of the total block copolymer, the unreacted poly(vinyl chloride), and the unreacted poly(acrylonitrile-co-butadiene) polymer in both reaction systems were obtained.