The catalytic cycle of the enzyme manganese peroxidase (MnP) requires the presence of dicarboxylic acids to chelate and stabilize the oxidized and very unstable Mn3+, which is responsible for the final substrate oxidation. However, the enzymatic degradation of an azo dye, Orange II, was successfully performed in the absence of any carboxylic acid. To analyze this possible discrepancy, the effect of the presence of several organic acids (oxalic, malonic, tartaric and citric acids) was studied on the kinetics and the extension of the degradation of Orange H. The Mn3+ chelating strength, an important factor that should influence the efficiency of the degradation, was determined for the different organic acids and the dye by cyclic voltammetry. Oxalic acid was determined to be the best chelator, followed by malonic, tartaric and finally, citric acid. Orange R was shown to act as a chelator, since the hydroxyl and sulfonic groups allow a stabilized complex to be formed, avoiding the use of any dicarboxylic acid. This distinctive property could be extended to other molecules with a potential binding capacity. (C) 2007 Elsevier Inc. All rights reserved.