A detailed investigation on the electrochemical reduction of oxygen was carried out using cyclic voltammetry at a glassy carbon electrode in dimethylsulphoxide (DMSO) containing 0.1 M tetrabutylammonium. perchlorate (TBAP) as the supporting electrolyte. A diffusion-limited, quasi-reversible one-electron peak corresponding to the oxygen-superoxide anion couple is seen. The quasi-reversibility was supported by the DeltaE(p) and I-p behavior with the square root of the scan rate, which is typical for a quasi-reversible process. We have used the theory developed by Nicholson for a study of the electrode reaction kinetics in order to measure a precise and reproducible value for the standard rate constant, k(0), of the quasi-reversible reduction of oxygen to superoxide anion. The k(0) mean value obtained was 9.3x10(-2) with a standard deviation of +/-0.4x10(-2) cm s(-1), and a coefficient of variation of 4.6%. The k(0) value obtained was compared with both a previous value found in the literature and with simulated curves. (C) 2003 Elsevier Science B.V. All rights reserved.