Electrical conductivity and the responses to SO2-N-2 mixtures of HCl-doped and CSA-doped solvent cast polyaniline films (PANI/HCl and PANI/CSA) were investigated. At a given doping level or with the same number of charge carriers present, electrical conductivity of PANI/HCl films is greater than that of PANI/CSA films due to the more closely packed crystalline domain of the PANI/HCl films which reflects its greater charge carrier mobility. PANI/HCl films do not respond to SO,, possibly because of its upper attainable apparent doping level or the number of charge carriers present. The increment in electrical conductivity of PANI/CSA films upon exposure to SO, depends critically on the apparent doping level and varies linearly with SO2 concentration. The lowest detectable concentration is 200 ppm and the responses to SO2 are reversible. (C) 2002 Elsevier Science B.V. All rights reserved.