Well-defined structures comprised of alternating microstripes of noble metals (An, Pt) and TiO2, covered with self-assembled monolayers (SAMs), are used to study long-range metallic effects on the photodegradation of the monolayers. It was found that under certain conditions, the presence of a metal in the vicinity of the photocatalyst does not increase its photoefficiency but, in fact, might reduce it significantly, in comparison with a bare photocatalyst. This effect depends on the size of the domains, as well as on humidity and the type of metal. It is suggested that although metal-induced charge separation is beneficial in promoting the production of hydroxyl radicals needed for the first steps of the degradation process, charge separation might also become detrimental under conditions that decrease the back-diffusion of reduced species, such as hydroperoxy radicals, needed at later stages of the degradation process.