A calorimeter was used to directly monitor the rate of emulsion polymerization of 3,4-ethylenedioxythiophene (EDOT) in aqueous solution for different concentrations of the initiator, potassium persulfate (KPS). The reaction involved polymerization and doping in one step, and yielded a poly(3,4-ethylenedioxythiophene) (PEDOT) with electrical conductivity. The electrical conductivity and yield of PEDOT increased as the molar ratio of KPS to EDOT increased until the molar ratio of unity. Then they decreased. The reaction rate was found to be governed by the concentration of the reactants: R-p proportional to [KpS](0.64) x [EDOT](0.16). The exponent of EDOT concentration had so low value that the reaction was of approximately zero order, which could be explained by the presence of a so-called degradative addition. In addition, the effect of sulfuric acid on the polymerization rate of EDOT was also investigated. The reaction rate reached the maximum at pH 2.7. (C) 2000 Elsevier Science Ltd. All rights reserved.