Electrochemical oxidation of some catecholamines such as dopamine (1), L-dopa (2) and methyldopa (3) has been studied in the presence of 4-aminobenzoic acid (4). Some electrochemical techniques such as cyclic voltammetry using diagnostic criteria derived by Nicholson and Shain for various electrode mechanisms and control led-potential coulometry were used. Variation in cathodic peaks due to the competition between intramolecular and intermolecular Michael addition reaction was studied in various pH values, and the best conditions for reaction with 4-aminobenzoic acid was obtained. The results indicate an ECE mechanism. The observed homogeneous rate constant (k(obs)) for Michael addition reaction was estimated by comparing the experimental cyclic voltammetric responses with the digital simulated results. Also, the Michael addition reaction rate constants (k(m)) were calculated using microscopic acidic dissociation constant of ammonium groups in 4aminobenzoic acid. The chemical oxidation reaction of catecholannines with periodate in the presence of 4-aminobenzoic acid was also studied spectrophotometrically. (C) 2005 Elsevier Ltd. All rights reserved.