The phase-transfer catalyzed polycondensation of alpha,alpha'-dichloro-p-xylene with 4,4'-isopropylidenediphenol was carried out using benzylethylammonium chloride in a two-phase system of an aqueous alkaline solution and benzene at 60 degrees C under nitrogen atmosphere. The rate of polycondensation was expressed as the combined terms of quaternary onium cation and 4,4'-isoprapylidenediphenolate anion rather than the fe ed concentration of catalyst and 4,4'-isopropylidenediphenol. The measured concentrations of hydroxide and chloride anion in the aqueous solution and cr,cr dichloro-p-xylene in the organic phase were used to obtain the reaction rate constant with the integral method, and to analyze the polycondensation mechanism with a cyclic phase-transfer initiation step in the heterogeneous liquid-liquid system.