The oxidative polymerization of aniline in formic acid (HCOOH) medium was investigated using formic acid of different strengths (5-25N). It was observed that a certain portion of the total polymer (similar to10%) remains soluble in the reaction medium. The UV-visible absorption spectra of the reaction mixtures in dilute and concentrated HCOOH indicate different initial reaction pathways and intermediates occurring under the conditions employed. It was found that the reaction yield and intrinsic viscosity of the polymer were dependent on the synthesis parameters. By contrast, the electrical conductivity was not sensitive to most of these variables. Elemental analysis revealed that the chemical composition of polyaniline (PANI)-formic acid salt is 4: 2.2, indicating two units of formate anions present per four-unit polymer chain containing 3:1 ratio of benzenoid to quinonoid moieties. Spectroscopic results showed that PANI prepared in HCOOH exists in an emeraldine oxidation state. A transmission electron micrograph of the soluble portion showed a wide distribution of spherical particles of different diameters in the range 20-200 nm. The thermal stability and crystalline structure of PANI-formic acid salt and its base form were ascertained by thermogravimetric analysis and wide-angle X-ray diffraction (WAXD) study, respectively. The base form of the polymer was stable up to 400degreesC and the acid form showed evolution of formic acid dopant in the temperature range 200-300degreesC. WAXD study showed a pseudo-orthorhombic crystal structure for PANI-formic acid salt. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91:991-999,2004.