Self-doped conducting polyaniline-modified indium tin oxide (ITO) electrodes were prepared by cyclic voltammetry on ITO substrates in aniline (AN) and o-aminobenzene sulfonic acid (OSA) mixed monomer solutions with AN/OSA mole ratios of 25/75, 50/50, and 75/25, followed by investigations on electrocatalytic activities of the copolymers to redox reactions of Fe(CN)(6)(3-/4-) as a probe in aqueous solutions of different pH using cyclic voltammetry. For a given pH, the P(25AN-co-750SA)-modified ITO electrode demonstrated the highest current density, followed by the P(50AN-co-500SA)- and by the P(75AN-co-250SA)-modified ITO electrodes. It can be concluded that a higher content of OSA (sulfonate) in the copolymer exhibited a higher extent of self-doping in the copolymer, leading to a higher electrocatalytic activity to redox reactions of the probe. The electrocatalytic activities of the copolymers decreased with increasing pH. The P(25AN-co-75OSA)-modified ITO electrode was electroactive for sensing the redox reactions of the probe in aqueous solutions of up to pH 7, the P(50AN-co-50OSA)-modified ITO electrode was electroactive for up to only pH 5, but the P(75AN-co-25OSA)-modified ITO electrode was not electroactive in aqueous solution of pH even as low as 2. (C) 2012 Elsevier Ltd. All rights reserved.