Abstract In situ infrared reflection spectra of thin polypyrrole films electrodeposited on platinum were obtained in aqueous KNO 3, KCl and KF solutions, within a wide potential interval ranging from the reduced, insulating state, to the oxidative degradation of the material. These spectra provide information on the molecular changes resulting from the generation of charge carriers (radical cations and dications) in this conjugated heterocyclic system, as well as the irreversible chemical changes accompanying its electrochemical oxidation at very positive potentials. The spectral region between 1000 and 1700 cm −1 contains information on the relative populations of charge defects as a function of potential, and the results confirm that electronic charge transport at moderate potentials is due to formation of radical cations, which oxidize further or recombine to form dications as the potential increases. At more positive potentials the polymer degrades with formation of carbonyl and hydroxyl groups as well as CO 2. The overoxidation process occurs at less positive potential in F − solution due to nucleophilic attack of OH − ions originating from dissociation of the aqueous solvent, as shown by the infrared spectra and previous calorimetric evidence. In the absence of irreversible chemical changes due to overoxidation, the infrared spectra obtained in the different solutions were similar, indicating weak interactions between ions and charge defects in the backbone.