Structural changes induced by electrochemical redox processes or by pH variations on conducting poly-3-methylpyrrole, electrochemically synthesized in NaClO4 acetonitrile solution, have been studied by infrared spectroscopy. With this aim infrared spectra of perchlorate doped poly3-methylpyrrole films at different oxidation states as well as after immersion in acid (pH= 1) and basic (pH= 12.6) aqueous solutions have been analysed. The existence of proton-doping mechanism in this polymer has been confirmed from the comparative study of spectra of oxidized/ reduced and acid/basic treated polymer. (c) 2007 Elsevier B.V. All rights reserved.