The electro-oxidation of carbon monoxide in perchloric acid at modified platinum electrodes dispersed in polyaniline (PAni) has been investigated by cyclic voltammetry. The results show that on a PAni/Pt-Sn electrode the oxidation of carbon monoxide is achieved at potentials as low as 0.2 V(RHE). Using a PAni/Pt-Sn rotating disk electrode allowed us to obtain an accurate value of the diffusion coefficient of CO in an aqueous electrolyte (HClO4).