Poly(3,4-ethylenedioxythiophene) (PEDOT) films were prepared from an aqueous solution by electrooxidation at different anodic potential in the range 0.8-1.5 V (vs. SCE). The effect of polymerization potential on conductivity, electrochemical behavior and ESR response of PEDOT film has been investigated. The overoxidation peak of PEDOT exists near the polymerization potential of 3,4-ethylenedioxythiopbene. The overoxidation behavior of PEDOT with polymerization potential yielded bell-shaped curve of the conductivity of PEDOT and the polymerization rate with the polymerization potential. This phenomenon has been reported for the first time. (C) 2003 Elsevier Science Ltd. All rights reserved.