Electrochemical oxidation of quercetin, as important biological molecule, has been studied in 0.1 M phosphate buffer solution, using cyclic voltammetry, chronoamperometry, rotating disk electrode voltammetry as well as quantum mechanical calculations. The heterogeneous charge transfer rate constant, k', transfer coefficient, alpha, and exchange current density, j(0). for oxidation of quercetin at the glassy carbon electrode are determined as 4.84 x 10(-2) CM s(-1), 0.65 +/- 0.01 and (1.17 +/- 0.39) x 10(-7) A cm(-2), respectively. The formal potential, E-0', of quercetin is pH dependent with a slope of -60.1 mV per unit of pH which is close to the anticipated Nernstian value of -59 mV for a two electrons and two protons process. The standard formal potential, E-0, of quercetin was found to be equal with 558 mV versus saturated calomel electrode (SCE). The mechanism of oxidation was deduced from voltammetric data in various pHs and also in different concentrations of quercetin. The diffusion coefficient of quercetin was calculated as 3.18 x 10(-6) cm(2) s(-1) for the experimental condition, using chronoamperometric results. The results of density functional theory (DFT) calculations for the oxidation of quercetin in aqueous solution, are also presented. The theoretical standard electrode potential of quercetin is obtained to be 568 mV versus SCE, which is in good agreement with the experimental value. The discrepancy between theoretical and experimental values is only 10 mV. The agreement verifies the accuracy of experimental method and the validity of mathematical model. (c) 2005 Elsevier B.V. All rights reserved.