Cu2ZnGeSe4 and Cu2ZnSnSe4 are quaternary semiconductors belonging to the adamantine compound family, contain only abundant elements, which makes these materials promising candidates for engineering on their base of different high-efficient and low-cost photovoltaic devices. CZTSSe solar cells with Ge alloying recently reached efficiency of 8.4%. By neutron powder diffraction our group could show that Cu2ZnSnSe4 and Cu2ZnGeSe4 occur in the kesterite and modified kesterite structure respectively. Moreover intrinsic point defects, like copper vacancies and Cu-Zn anti-sites, have been verified. Because these defects have a critical influence on the electronic properties of the material, detailed knowledge on point defect type and concentration is crucial. The aim of the experiment is to get correlated information about changes in lattice parameters and cation site occupancies of Cu2Zn(Sn1-xGex)Se4 over the whole range of x from 0 to 1.