A detailed mechanism is proposed concerning the initial stages (up to the formation of the dimer) of the electropolymerization of ortho-phenylenediamine (oPD) to yield the polymer (PPD). Cyclic voltammetry measurements have been performed aiming to single out the effect of pH and to determine the number of exchanged electrons. The experimental evidence has been modelled considering 10 possible monomers (as obtained from the combination of oxidation and protonation/deprotonation reactions) involved in nine reactions to produce five different dimer species. Such a modelistic picture developed starting from the experimental findings, collected both in this paper and as found in the literature, is tightly compared with theoretical calculations. In particular, both E-0 values and thermodynamic quantities (Delta U, Delta H and Delta G variations), relating to the reaction path leading to the formation of the dimer, have been determined at the ab initio level of the theory (molecular structures relevant to transient complex species and transition states, as well as reaction free energy variations and activation energies were calculated too). (c) 2005 Elsevier B.V. All rights reserved.