The poly(o-aminophenol) (POAP) redox process has been studied in aqueous acid solution using spectroscopic and optic in situ techniques. The redox transition of the POAP from its completely oxidized state to its completely reduced state occurs through two consecutive reactions in which a charged intermediate species takes part. UV-Vis and Raman signals agree with an increase of the concentration of an intermediate species until the potential of the maximum redox peak, which later diminishes with the potential. The results of in situ FTIR spectroscopy agrees with Raman measurements. Probe beam deflection (PBD) suggests that during its oxidation, the polymer incorporates anions in a first process and then expels protons in a second one. (C) 2004 Elsevier B.V. All rights reserved.