The phase-transfer radical copolymerization of acrylonitrile (AN), and methylacrylate (MA) with tricaprylylmethylammonium chloride was investigated in Na2S2O8 aqueous-organic two-phase system at 25 degrees C and under nitrogen atmosphere. The rate of copolymerization was expressed as th combined terms of quarternary ammonium ion and peroxydisulfate ion in the aqueous phase rather than the fed concentrations of catalyst and Na2S2O8. The observed initial rate of copolymerization was used to analyze the copolymerization mechanism with a cyclic phase-transfer initiation step in the heterogeneous liquid-liquid system. The monomer reactivity ratio r(1) and r(2) obtained from the analysis of copolymerization mechanism were 1.584 and 0.856, respectively.