We report a spectroelectrochemical study in the wavelength range 300 nm < lambda < 800 mm, of poly (o-aminophenol) (POAP) in aqueous acid solutions of different pHs and with different supporting electrolytes. Four features were observed in the absorption spectra : the uv tail of the band of the aromatic ring structures, a weak absorption band at 340 nm corresponding to the polymer in the reduced state, a strong broad band at 440 nm corresponding to the oxidized state of the polymer and a maximum in absorbance at about 750 nm when going from the reduced to the oxidized states. Cycling the potential while holding lambda at 750 nm yields a transient response in the absorbance peaking at the peak potential of the voltammetric response, which also is the potential of maximum conductivity of POAP. From the absorbance data the number of exchanged electrons during the oxidation cycle was 0.4 which agrees well with that estimated from the oxidation charge and the polymer thickness. This indicates that one in every four or five monomer units is being oxidized. The dependence of the absorbance on pH shows that as the pH increases the protonation of the amino groups decreases. The experimental results suggest that the imino groups in the oxidized state are not protonated. This is in agreement with the fact that POAP is easily oxidable and a very poor conductor. This is also in agreement with the increasing difficulty in reducing the polymer as the pH increases. Hysteresis effects were observed both in the voltammogram and in the absorbance during potential cycling. In the presence of anions bigger than perchlorate, such as benzenesulphonate, both the oxidation charge and the absorbance change decrease.