Abstract Details of the synthesis of ABS copolymers in the presence of potassium persulphate and tertiary dodecylmercaptan in an emulsion stabilized with the potassium soap of disproportionated rosin are determined by the occurrnce of the copolymerization of styrene with acrylonitrile both in polybutadiene seed particles and also in newly formed particles. It is shown that the order of the reaction with respect to the initiator (0.44) and that with respect to the emulsifier (0.97) depend only on the rate of copolymerization in new particles. The high order with respect to the emulsifier arises from the effect of its concentraton not only on the number of new particles but also on the average number of the radicals in the particles. The presence of tertiary dodecylmercaptan in the system displaces the process to new particles, which leads to the displacement of the monomers from the seed to the new particles and to a reduction in the efficiency of grafting of the monomers on to the rubber. Approaches are put forward to raise the efficiency of grafting of ABS copolymers, the approaches being based on the separation in time of the processes involved in the formation of the graft and the matrix copolymers.