Interpenetrating organic polymer composites consisting of two conducting polymers have been successfully prepared by sequential electropolymerization of the parent monomers: aniline and 3,4-ethylenedioxythiophene. The electropolymerization of poly(3,4ethylenedioxythiophene) (PEDOT) and polyaniline (PANI) was carried out in acetonitrile solution. In the first step, a potentiostatic electrodeposition of PEDOT was performed. Then in the second step, a potentiostatic electrodeposition of PANI was carried out, in acetonitrile solution without addition of protic acid, in the potential range where PEDOT was in its oxidized and conducting states. We observed that the oxidized PEDOT film exhibited a Lewis acid behavior that allowed the polymerization of polyaniline on PEDOT modified electrode in the absence of protic acid. A mechanism of the polymerization of PANI onto PEDOT was proposed. The electrochemical properties of the composite films have been analyzed by means of cyclic voltammetry and in situ resistometry. In the case of the interpenetrating composite based on PEDOT/PANI, the variation of the electronic resistance as a function of potential displayed a drastic change comparing to that of PEDOT or PANI alone. These results indicated that the obtained electroactive material was not a bilayer or sandwhich structure but an interpenetrating composite in which the two conducting polymers interacted each other to give new electrical properties. Also, we observed an increase of the stability of PANI in the PEDOT/PANI composite when the measurements were carried out in acidic aqueous solution. An important advantage of these organic conductive polymer composites is the simplicity of their preparation. (c) 2005 Elsevier B.V. All rights reserved.