Light harvesting chalcogenide materials have strong potential applications for photovoltaic due, in part, to the ability of their structures to accommodate shift from the ideal stoichiometry. This study is devoted to the chemical and structural investigations of two specific series of materials, Cu(In, Ga)Se2 (CIGSe) and Cu2ZnSnS4 (CZTS). Both of them receive currently a strong incentive in the domain of thin film solar cells. On the basis of accurate chemical analyses, conventional powder and single-crystal X-ray diffraction, resonant X-ray scattering with synchrotron radiation the capacity of the chalcopyrite and kesterite structures of CIGSe and CZTS to accommodate deviations from the stoichiometry is discussed. Formally, the former is found to be more flexible than the latter even if this one can self adapt to copper-poor and copper-rich compositions without any structural change except in terms of the cation distributions.