Polyaniline (PANI)-poly(2-acrylamido-2-methyl-1-propanosulfonic acid) (PAMPS) composite thin films are obtained by a chemical bath deposition method on conductive glass substrates. The incorporation of the polyacid in PANI, demonstrated by atomic force microscopy, ultraviolet-visible spectroscopy and Fourier transform infrared spectroscopy studies, causes changes in the electronic structure of PANI and gives a different microstructure when compared to the unmodified PANI film. The lower optical transition band gap as well as an oriented fibril microstructure of the composite film give rise to a decrease in the redox potentials of PANI, favoring a lower optical switch voltage for the PANI-PAMPS composite film-based solid electrochromic devices. (C) 2003 Elsevier Science B.V. All rights reserved.