The oxidation of ascorbic acid in citrate buffer at pH 2.9 has been investigated with and without added copper ions as catalyst, and the loss of ascorbic acid followed by polarographic analysis. Certain flavonoids occurring in blackcurrant juice were used as antioxidants and their activity, in the absence of added copper ions, was found to decrease in the order, quercetin and dihydroquercetin (equal), kaempferol and rutin. Cyanidin‐3‐rhamno‐glucoside and delphinidin‐3‐glucoside accelerated the oxidation. In the presence of added copper ions quercetin showed increasing antioxidant activity up to 12.5 � 10^‐5M (7.8 : 1, quercetin‐copper ratio) after which the activity decreased rapidly. Anthocyanins were shown to possess slight antioxidant activity in the presence of copper ions. the probable mechanism of ascorbic acid oxidation is outlined, and the mode of action of flavonoids as antioxidants discussed.