Abstract Colloidal dispersions of platinum and palladium protected by nonionic surfactants were prepared by photo-irradiation. They work as active catalysts for the hydrogenation of unsaturated fatty acids and their sodium salts. The regio-selectivity was observed in the hydrogenation in the organized media which surround the colloidal particles. Thus, 10-undecenoic acid was more rapidly hydrogenated than 2-undecenoic acid.