The redox processes of quercetin in acidic medium (pH 1.8) were investigated on a graphite-wax electrode by cyclic voltammetry, potential-controlled UV/Vis spectra and derivative cyclic voltabsorptometry. A long-optical-path thin-layer cell was used for the spectroelectrochemical measurements. A stable quinonic product with a united conjugated structure was found to form at the first anodic peak or higher potentials via an EC mechanism, and was non-electroactive unless oxygen evolution potential was reached. Quercetin both in the free solution and pre-adsorbed on electrodes participated in the reaction at the first anodic peak, and the contribution of species in solution to total current was estimated. The subsequent anodic peaks as well as all the cathodic peaks were adsorption waves corresponding to the further redox of the adsorbed intermediates. The results demonstrate that cyclic voltabsorptometry in a long-optical-path cell is a feasible method for the analysis of electrochemical processes coupled with adsorption and homogeneous chemical transitions.