Polypyrrole (PPy) was deposited into nanostructured porous silicon (PS) layers by the electropolymerization of the corresponding pyrrole monomer in an acetonitrile solution. The potential or current transients recorded during the polymerization process exhibit different characteristic stages: preferential nucleation of PPy at the pore bottom with a partial oxidation of PS, polymer growth inside the pores, and polymerization on the outer surface of PS. Accordingly, a ternary nanostructure of PS/SiO2/PPy is obtained. It is very promising that a significant increase in electrical conductivity of this PS/PPy junction is observed compared to a bare PS layer, especially in lightly doped PS. In this way, complete oxidation of the PS layer is not expected during the polymerization, and polymer deposition may even partially inhibit the PS oxidation. Furthermore, the deposited PPy film could be electrochemically driven between the oxidized (conducting) form and the reduced (insulating) form with an observable color change in the film from dark brown when oxidized to faint yellow when reduced. Results of the characterization of this heterosystem, uniformity of the PPy within the pores, and the pore-filling phenomena using different surface analytical methods are presented and thoroughly discussed. (c) 2006 The Electrochemical Society.