Ascorbic acid and dehydroascorbic acid kinetics in model solutions, under oxidizing and reducing conditions, were determined using a first order reversible consecutive reaction model. The time-dependent changes for both ascorbic acid and dehydroascorbic acid were strongly correlated with the model described here. Presence of reducing or oxidizing agents in the medium affected the reaction rate constants. In control, the rate constant of reversible reaction in which dehydroascorbic acid reduces to ascorbic acid (k(2)) was found to be almost zero, whereas the rate constants for oxidation of ascorbic acid to dehydroascorbic acid (k(1)) and hydrolysis of dehydroascorbic acid to 2,3-diketogluconic acid (k(3)) were close to each other. Addition of cysteine, as the reducing agent, into the medium significantly increased (k(2)). Increasing the cysteine concentration also increased (k(2)). Nevertheless, addition of Fe3+ as the oxidizing agent, into the medium had no effect on (k(2)), but significantly increased k(1) and k(3). (C) 2007 Elsevier Ltd. All rights reserved.