Solid electrochromic devices (ECDs) are prepared with chemically deposited electroactive polyaniline (PANI) and PANI-poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) thin films. The analysis of their electrical current responses suggests a higher redox potential value of the unmodified PANI thin film compared to that of the PANI-PAMPS ECD. Optical transmittance spectra of the devices show that the polyacid incorporation in PANI thin films reduces the switching voltage from +/- 2 to +/- 1.5 V, increases the optical contrast ratio by 10% at around 650 nm and doubles the optical response rate of the devices using LiClO4 dissolved in polymethyl methacrylate (PMMA) as a solid electrolyte. A parametrical analysis of the optical kinetic curves, together with the Butler-Volmer equation, indicates that PANI films contain two kinetically different phases: an interconnected network on which electrical conduction is predominated by percolation and a diffusion-controlled charge transfer zone. The first one contributes to the faster term and the second one to the slower term of the polymeric ECD optical responses. (C) 2003 Elsevier B.V. All rights reserved.