The aim of this work was to study the effect of thermal treatment (vacuum or Ar) on the main properties of CuIn1-xGaxSe2 (CIGS) thin films prepared by selenization of identical metallic alloys. All the CIGS films showed chalcopyrite structure with preferential orientation in the (112) plane. Ar thermal treatment led to better crystallinity, lower electrical conductivity and an increase in the grain size. It has been found that the Se diffusion and the distribution of the elements depend on the selenization atmosphere. After Ar selenization, an increase in band gap energy of 0.09 eV was observed, which is associated with a reduction in defects. (C) 2004 Elsevier B.V. All rights reserved.