Electrochemical oxidation of quercetin has been studied in the absence and presence of benzenesulfinic acids as nucleophiles in an acetonitrile+water mixture, using cyclic voltammetry and controlled-potential coulometry. The results indicate that the o-quinone derived from quercetin participates in a Michael addition reaction with the benzenesulfinic acids to form the corresponding sulfonyl derivatives. The electrochemical synthesis of these derivatives has been successfully performed at carbon rod electrodes in an undivided cell in good yield and purity. (C) 2003 Elsevier Science B.V. All rights reserved.