Copper metal nanoparticles have been formed by irradiation with 253.7 nm light from a low pressure Hg-arc lamp in the presence of a protective agent poly(N-vinylpyrrolidone). The role of a photo-sensitizer, benzophenone (BP), in the formation of Cu metal particles was studied. The nanoparticles have been characterized by their absorption maxima and transmission electron micrographs. The average particle size for Cu was 15 nm. It appears that the presence of BP is essential for the photochemical preparation of copper nanoparticles. Laser spectroscopic studies have revealed that BP ketyl radical does not participate in the formation of Cu metal nanoparticles. (C) 2003 Elsevier Science B.V. All rights reserved.