Polyaniline (PANI) was prepared by the oxidative polymerization of aniline hydrochloride and aniline sulfate with ammonium peroxydisulfate in acidic aqueous medium. Aniline was similarly oxidized in solution of phosphoric acid. Thin PANI films are produced at the interfaces with the reaction mixture. The progress of polymerization and the PANI-filin formation were monitored in situ by Fourier transform infrared spectroscopy after depositing a droplet of the reaction mixture on a ZnSe crystal. The influences of chloride, sulfate, and phosphate counter-ions on the initiation and progress of aniline polymerization have been analysed. The spectra of the completed PANI films protonated with various acids have been compared with the spectra of the corresponding PANI bases obtained after deprotonating the films with ammonium hydroxide. The peaks in the region 3400-2800 cm(-1), so-called H-peaks, have been analyzed in detail. The influence of chloride, sulfate, and phosphate counter-ions on the shape and position of some of the peaks has been noted. (c) 2006 Elsevier B.V. All rights reserved.