The electrochemical impedance spectroscopy of a platinum-modified polyaniline film formed by cycling potential or constant potential deposition of platinum particles in polyaniline film is studied in sulfuric acid solution. The results show that compared with the pure polyaniline film [ H2SO4 solution interface, the deposition of platinum in the polyaniline matrix greatly reduces the electrochemical charge transfer resistance (R-ct) in the platinum-modified polyaniline film ] H2SO4 solution interface. The surface morphology of the electrodes is observed by scanning electron microscopy. This shows that the different polarization modes affect the nature of the platinum deposits, such as their size, amount, real surface area and distribution in the polymer matrix. The electrocatalytic activity of platinum-modified polyaniline film electrodes for methanol oxidation is discussed on the basis of platinum loading and real surface area which result from varying electrodeposition modes, such as potential limits in cyclic potential polarization or selected potentials in constant potential polarization. Finally, the relationship between impedance characteristic and electrocatalytic activity has been suggested. (C) 2003 Elsevier Science B.V. All rights reserved.