Structures of compounds in the Cu 2 Se-In 2 Se 3 -Ga 2 Se 3 system have been investigated through X-ray diffraction. Single crystal structure studies for the so-called stoichiometric compounds Cu(In,Ga)Se 2 (CIGSe) confirm that the chalcopyrite structure (space group I4 ¯2d) is very flexible and can adapt itself to the substitution of Ga for In. On the other hand a structure modification is evidenced in the Cu 1 À z (In 0.5 Ga 0.5 ) 1 + z/3 Se 2 series when the copper vacancy ratio (z) increases; the chalcopyrite structure turns to a modified-stannite structure (I4 ¯2m) when zZ 0.26. There is a continuous evolution of the structure from Cu 0.74 (In 0.5 Ga 0.5 ) 1.09 Se 2 to Cu 0.25 (In 0.5 Ga 0.5 ) 1.25 Se 2 ((i.e. Cu(In 0.5 Ga 0.5 ) 5 Se 8 ), including Cu 0.4 (In 0.5 Ga 0.5 ) 1.2 Se 2 (i.e. Cu(In 0.5 Ga 0.5 ) 3 Se 5 ). From this single crystal structural investigation, it is definitively clear that no ordered vacancy compound exists in that series. X-ray photoemission spectroscopy study shows for the first time that the surface of powdered Cu 1 À z (In 0.5 Ga 0.5 ) 1 + z/3 Se 2 compounds (z a 0) is more copper-poor than the bulk. The same result has often been observed on CIGSe thin films material for photovoltaic applications. In addition, optical band gaps of these nonstoichiometric compounds increase from 1.2 to 1.4 eV when z varies from 0 to 0.75.