The ac-impedance spectroscopy of the electro-oxidation of 2-propanol and methanol was investigated in a H2SO4 solution using a carbon-supported platinum catalyst as a function of potential between 350 and 750 mV (vs. SHE) at a variety of temperatures between 303 and 353 K. It is found that the impedance spectrum of 2-propanol is very similar to that of methanol. Semicircles that have induction loops at the low frequency ends were observed in complex plane plots at high potentials between 600 and 750 mV in both the cases of 2-propanol and methanol, which suggests that the rate-determining steps of the electro-oxidation of intermediates adsorbed on platinum should be involved in their reaction processes. Assuming appropriate models involving the adsorbed intermediates for 2-propanol and methanol, the impedance spectra were explained successfully. The impedance analysis provides the values of reaction parameters concerning a coverage relaxation time for an intermediate and a charge-transfer resistance, which make it possible to compare the reaction rates of the intermediates between 2-propanol and methanol. The reaction parameters suggest that a consecutive reaction involving the intermediate for 2-propanol proceeds more slowly than that for methanol, and also suggest that a direct reaction pathway from 2-propanol to acetone, which does not go through the intermediate, exists in parallel. Additionally, the influences of acetone concentration on the coverage relaxation time and the charge-transfer resistance for the electro-oxidation of 2-propanol were also discussed. (C) 2004 Elsevier B.V. All rights reserved.