The electrochemically prepared polyaniline-12-molybdophosphoric acid (P12MPA) thin films in conducting emeraldine salt form of polyaniline (PAni) matrix is verified by the presence of redox peaks in the UV-vis spectra for different dopant concentrations. The optical band gap energy in these films is dopant concentration dependant and tunable. From the FUR spectra, it is evidenced that, the characteristic peaks of PAni is present in P12MPA thus there is only conformational change due to the inclusion of molybdophosphoric acid (12MPA) in the polymer matrix. The amorphous nature of the film is proved from the XRD. XPS peaks of PAni and P12MPA thin films have shown the evidence for the presence of quinoid imine, benzenoid amine and the polaronic nitrogen and the oxidation level is quantitatively analysed. P12MPA has the oxidation level of around 51% which matches with that of protonated emeraldine form. Due to the inclusion of 12MPA, in addition to para coupling of the monomer, ortho coupling also takes place. Impedance analysis shows the role of temperature in crosslinking of polymer chain and dopant anions. The high response of P12MPA (64.8%) towards acetone vapor provides evidence for improved sensing behaviour of the doped polymer. (C) 2010 Elsevier B.V. All rights reserved.