Electrodeposition of ZnO/eosin Y hybrid thin films from aqueous mixed solution of zinc chloride and eosin Y as promoted by reduction of oxygen has been studied. Highly oriented crystalline hybrid thin films with two distinctive structures have been obtained depending on the redox state of eosin Y. Deposition at potentials more positive than that of eosin Y reduction resulted in a formation of compact ZnO crystals into which eosin Y molecules are entrapped, while that accompanied with the reduction of eosin Y yielded a film consisting of sponge-like ZnO crystals with internal nanoporous structure to which eosin Y molecules are adsorbed. The addition of eosin Y accelerated the film growth both in oxidized and reduced forms due to its catalysis toward the reduction of oxygen. Photoelectrochemical measurement in an I-/I-3(-) redox electrolyte solution revealed that the hybrid thin film in the latter structure performed as an efficient sensitized photoelectrode because of its porous nature. (C) 2003 The Electrochemical Society.