Polycrystalline CuGaSe2 (CGS) films with slightly Ga-rich composition were prepared on Mo/soda-lime substrates by the 'bilayer' process. The film surfaces were modified by chemical bath treatment with In-2(SO4)(3), thioacetamid, and triethanolamin to improve the performance in solar cell applications. The film compositions were characterized by X-ray fluorescence and the surface of treated films was investigated by X-ray photoelectron spectroscopy (XPS). Solar cells with ZnO/CdS/CGS/Mo/soda-lime glass structure were fabricated, and the current-voltage properties and the quantum efficiency were analyzed. Improvement of the spectral response, especially in the long wavelength region, was observed for the samples treated with the chemical bath, which results in increase in a short circuit current density. An increase in the parallel and series resistance of the cells was also observed with the treatment. The surface compositions of the CGS thin films modified by the chemical bath are discussed on the base of the results of XPS. (C) 2003 Elsevier Science B.V. All rights reserved.