We report here the first electrochemical polymerization of polypyrrole (PPy) on gold substrates in aqueous solutions containing additives of 1 mM rutile TiO2 nanoparticles under the irradiation of UV light. As a result, PPy/TiO2 composites were prepared with an increased conductivity of 135 S cm(-1), raising one order as compared with that of pure PPy. Furthermore, the thermal stability of the PPy-based composites was improved, as shown from thermogravimetric analysis (TGA). The effects of TiO2 on the PPy electropolymerization under UV irradiation were examined by its nucleation and growth mechanism and the onset potential of polymerization. The results indicate that the nucleation and growth are instantaneous two-dimensional and three-dimensional processes before and after nuclei overlapping, respectively, for the composite electropolymerization, which are distinguishable from those of pure PPy electropolymerization before nuclei overlapping. (C) 2004 Elsevier B.V. All rights reserved.