This paper describes the results of spectroelectrochemical studies performed using thin films composed of polypyrrole (PPy) doped with either para-toluenesulfonate (pTS) or poly-2-methoxyaniline-5-sulfonic acid (PMAS). Both films displayed changes in their absorption spectra consistent with reduction of the oxidised form of polypyrrole (PPy), when subjected to negative potentials after immersion in acetonitrile solutions containing tetrabutylammonium perchlorate (TBAP). Surprisingly, at even more negative potentials, both films showed a further set of spectral changes including the reappearance of a free carrier tail at wavelengths beyond 600 nm. These spectral changes suggest that the conductivity of the films was increasing at very negative potentials, a conclusion supported by in situ resistometry experiments performed using PPy/pTS films. These additional spectral changes were not observed during spectroelectrochemistry experiments performed using PPy films immersed in aqueous solutions, or organic solvents that had previously been de-aerated. These observations, as well as other evidence, suggest that molecular oxygen plays an important role in the occurrence of the additional spectral changes observed at very negative potentials. (C) 2009 Elsevier B.V. All rights reserved.