The electrocatalytic oxidation of carbon monoxide has been investigated on platinum-modified polyaniline (PAni) film electrodes ill sulfuric acid solution. The results show that polyaniline significantly increases the catalytic activity of platinum nanoparticle for the oxidation of carbon monoxide in comparison with polycrystalline bulk platinum. The oxidation potential of CO is 0.65 V in platinum-modified polyaniline electrode (where the platinum mass is 12.1 mu g, film thickness is 0.15 mu m), while the potential is 0.82 V in bulk platinum electrode. The effect of the increase in the catalytic activity may be due to the high dispersion of platinum nanoparticles incorporated into polyaniline film and the large surface area of polyaniline film. It is found that the electrocatalytic activity depends on the amount of deposited platinum, the loading sequence of platinum and polyaniline, the platinum loading procedure, the medium temperature, and the concentration of sulfuric acid. The film thickness of polyaniline appears to have no effect towards the electrocatalytic oxidation of carbon monoxide. (c) 2005 Elsevier B.V. All rights reserved.