Sulfide chalcopyrites Cu(In, Ga)S-2 (CIGS) have been proposed as top cell for tandem solar devices with silicon as bottom cell. In that context, the knowledge of the Cu2S-In2S3-Ga2S3 pseudo-ternary system is of prime importance. This study presents an overall investigation of this system on the basis of a chemical crystallographic approach on bulk materials. Particular attention was paid on the copper-poor side of the so-called stoichiometric CuInS2-CuGaS2 line. Based on our investigations, it turns out that the sulfide CIGS system is much more structurally complex than the selenide Cu(In, Ga) Se-2 one (CIGSe). Especially, the adaptability of the chalcopyrite structure toward Cu deficiency is significantly lower for the sulfides than for the selenide counterparts. Correlatively, a large 2-phase domain exists between the CuInS2-CuGaS2 line and the very copper-poor region (Cu(In, Ga)(3)S-5 and Cu(In, Ga)(5)S-8). Additionally, the crystal structures of these very copper-poor compounds contain [InS6] octahedra and then are totally different than the chalcopyrite structure. These features could explain why CIGS-based-solar cells are, up to now, less efficient than the CIGSe ones.