The thermal degradation of polyaniline films polymerized in situ on silicon windows, and protonated with hydrochloric, sulfuric, or phosphoric acid, has been studied by FTIR spectroscopy. The conductivity decrease and spectral shifts induced by thermal ageing at 150 degreesC have been examined and are discussed in terms of deprotonation. Other chemical changes and the nature of the structural modifications introduced during thermal treatment have been interpreted with the help of FTIR spectroscopy. (C) 2004 Elsevier Ltd. All rights reserved.