Effects of metal ions and flavonoids on the oxidatio of ascorbic acid (I) have been investigated polarographically in the acetate buffer solution of pH 5.4. since I gives a diffusion-controlled anodic wave due to a two-electron oxidation to form dehydroascorbic acid, the rate of the oxidation of I can be followed by measuring the change in the limiting current of I with time. The rate of the oxidation of I was enhanced by the presence of some metal ions. Among them, the effect of copper (II) was especially pronounced. Such the fact can be explained on the basis of the formation of a coordinative linkage between metal and ascorbate ions and the reducibility of metal ions. Contrary to this, flavonoids exhibited an inhibition effect on the copper (II)-catalyzed oxidation of I. The effect became more marked in the order of 3-hydroxyflavone<rutin<quercetin, but no inhibition was observed for flavone. From the experimental result that the complex forming tendency of copper (II) with flavonoids increase in the order of flavone<3-hydroxyflavone<rutin<quercetin, a decrease in the concentration of free copper (II) as a result of the complex formation between flavonoids and copper (II), was found to be primarily responsible for the inhibition effect of flavonoids.