The kinetics of uncatalysed and Cu(II) ion-catalysed oxidation of ascorbic acid by molecular oxygen in the presence of citric acid and its anions were studied at 25-degrees-C. In the pH range investigated (2.5-6.0), the rate of cupric ion-catalysed oxidation was found to be first order with respect to ascorbic acid. The observed effects of pH and cupric ion concentration on the reaction rate suggests a mechanism which may involve the formation of a transition complex between monoascorbate ion and Cu(II)-citrate chelate. The catalytic ability is in the order Cu(II) > H3CiCu+ > HCiCu- > CiCu-2. Hydrogen peroxide was also detected during the reaction.