Perovskite LaNiO3 nanoparticles were prepared by a reverse (water/oil) microemulsion method. The microemulsion system consisted of cetyltrimethylammonium bromide, 1-butanol, cyclohexane, and the metal salt solution. The procedure was carried out using aqueous ammonia as the co-precipitating agent. Reverse microemulsions were characterized by measuring the conductivity of the solutions. The dried precipitate was transformed to a perovskite by calcination at 750 degrees C. Nanosized particles were characterized by thermal analysis, X-ray diffraction, UV-diffuse reflectance spectroscopy, and transmission electron microscopy. The LaNiO3 samples have narrow band gaps and consequently achieved visible-light-induced photocatalytic activity towards the degradation of naphthalene species. The solar photocatalytic activity was found to be influenced by the nanoparticle size. (C) 2010 Elsevier B.V. All rights reserved.