The anodic oxidation of o-phenylenediamine at a platinum electrode in aqueous buffers at different pH (1−7), where growth of very thin (few nanometers thickness) and self-sealing polymeric films occurs, was investigated by cyclic voltammetry (CV) combined with off-line electrospray ionization-ion trap mass spectrometry (ESI-ITMS). Soluble oligomers (dimers, trimers, and tetramers in different oxidation states) formed in the course of the anodic oxidation were detected/identified by electrospray ionization-ion trap sequential mass spectrometry (ESI-ITMSn, n = 1−5) while their relative abundance could be estimated from full-scan ESI-MS spectra of the quasi-molecular ions. In particular, oligomer abundances and the abundance ratios between different n-mers of the same class (e.g., structures corresponding to the same value of n but having different m/z ratios) were considered, and their variation with pH and potential range adopted for polymerization was studied. A mechanistic pathway for the growth of poly(...