The electropolymerization of pyrrole, N-methyl-pyrrole, 3-methyl-thiophene and aniline in the presence of Keggin-type heteropolyacids (HPA)such as (SiW12040)4-, (PW12040)3- and (PMo12040)3- gave rise to conducting films on o the electrode. Considering their enormous size (~ = i0 A), these HPAs cannot be expelled during the reduction of the polymer ; on the contrary, at cathodic potentials, they undergo several electronic transfers. Moreover, even scattered in these films, they exhibit important catalytic activities towards oxygen and proton reduction. Complexation between these HPA and the various monomers may contribute to the excellent electropolymerization which provides materials of igh density and good conductivity. The intervention of the HPAs in the conductivity mechanism cannot be excluded when explaining the relatively good conductivity in spite of the long interchain distances.