Abstract Conducting poly(o-aminophenol) films (PoAP) have been electrosynthesized onto Pt substrates by cyclic voltammetry from oAP in acidic solution (HClO4 0.1M/KClO4 0.1M, pH 1.1). The electrosynthesis mechanism underlying the electrochemical oxidation of oAP has been investigated by in situ (Cyclic Voltammetry (CV) and Electrochemical Quartz Crystal Microbalance (EQCM)) and ex situ (X ray Photoelectron Spectroscopy (XPS)) techniques. In depth studies were performed through the characterization of polymeric layers grown up to 5, 8 and 125 voltammetric cycles. Following the initial formation of radical cations and their coupling, an induction period was noticed during which no net mass deposition occurs but rather a chemical rearrangement of adsorbed linear dimers to give aminophenoxazine like closed structures. Our experimental findings evidenced the necessity to completely reduce the linear dimer in order to allow its cyclization which is preliminary to polymer formation. We assumed that aminophenoxazine units initiate polymerization and then the addition of oxidized monomers gradually becomes the predominant route for polymer chains propagation. After 125 cycles a thickness of about 54-62nm was estimated by EQCM. XPS investigation supported the presence of linear dimers during the induction period and allowed to attribute unambiguously a phenazine-like closed structure to the finite film. The reduced state of PoAP was characterized with nitrogen functionalities showing binding energies intermediate between fully protonated and neutral ones. The presence of carbonyls terminal groups was also stated together with hydrating water. Heating experiments in ultra-high vacuum (UHV) conditions have revealed water molecules so strongly entrapped along the polymer chain through hydrogen bonds to be removed only at 200°C.