The early oxidation process (EOP) of dopamine (DA) was investigated by cyclic voltammetric measurements under various experimental conditions (e.g., the rate, direction and number of potential scan, the solution pH and the concentration of DA were changed) and quantum chemical calculations. The oxidation of 5-methyldopamine (5-MDA) was also examined for comparison with the EOP of DA. The EOP of DA proceeds in an so-called ECC mechanism, i.e., a 2e- - 2H(+) process followed by an intracyclization of the oxidized form of DA (i.e., dopamine o-quinone) through an unprotonated amino group, which forms a leucodopaminechrome, and furthermore a homogeneous electron-exchange reaction between dopamine o- quinone and leucodopaminechrome, forming dopaminechrome. The redox reaction of the leucodopaminechrome/dopaminechrome couple was also observed. On the other hand, the electro-oxidation of 5-MDA is simple, i.e., a 2e- - 2H(+) process without such follow-up chemical reactions because the 5-position of its benzene ring is occupied with methyl group and consequently such an intracyclization as in the case of DA does not occur. The cyclic voltammetric results obtained for DA and 5-MDA were reasonably supported by the quantum chemical calculations for DA, some products its oxidation produces and 5-MDA. (C) 2016 Elsevier Ltd. All rights reserved.