Relations between structure, antioxidant capacity and formal redox potentials (E°') of six flavonoids dissolved in acetonitrile were studied by cyclic voltammetry and square wave voltammetry using a glassy carbon electrode. This work shows correlation between the electrochemical parameter E°' and the number of hydroxyl groups present in the B ring of flavonoids. Besides, it has been observed that E°' measured as a function of B ring oxidation depends considerably on the functional groups present in the ring C of these molecules. Since antioxidant activity of flavonoids is attributed to their proton-donating, the results of this work were compared to previous reports regarding the antioxidant capacity of these compounds. Even though the antioxidant activity of flavonoids depends on several factors, the conclusions of this investigation suggest that the E°' value should be considered as a key parameter for the evaluation of antioxidant activity.