Sequential doping of polyaniline base in the oxidation state of emeraldine (abbreviated as PANI) first with iron(III) chloride and then treated with hexafluoroacetylacetone (HFAA) is described. The results obtained by using a combination of spectroscopic techniques (UV-vis-NIR, mass spectroscopy, Mossbauer effect spectroscopy, EPR) unequivocally show that complete Lewis acid-type complexation of PANI with FeCl3 occurs only in the solid state, i.e. after removal of the solvent. In the solution an equilibrium is established between PANI complexed with FeCl3 and FeCl3 complexed with nitromethane. The addition of HFAA to the solution, being in equilibrium, transforms Lewis acid doped PANI into mixed doped polymer, which upon casting and solvent removal gives a solid material of a general formula: PANI(FeCl3)(x)(HCl)(y)(HFAA)(z). In this compound FeCl3 is complexed with amine nitrogens, imine nitrogens are protonated with HCl. HFAA, dispersed in the polymer matrix, serves as a plasticizer. The determined chemical constitution is a direct consequence of the reaction of HFAA with FeCl3 complexed on PANI imine sites to give HCl and iron(III) hexafluoroacetylacetonate. The former protonates the imine sites whereas the latter is removed from the system by extended pumping as proved by mass spectroscopy. Mossbauer spectra unequivocally show that FeCl3 complexed on amine sites remains intact. Films of PANI(FeCl3)(x)(HCl)(y)(HFAA)(z) show room temperature conductivity of ca. 3 x 10(-3) S cm(-1) and improved mechanical properties as compared with PANI complexed solely with FeCl3 due to HFAA plasticizing effects. (c) 2005 Elsevier B.V. All rights reserved.