The atomic structure of the potential photovoltaic materials Cu(2)ZnSnS(4) (CZTS) and Cu(2)ZnSnSe(4) (CZTSe) is discussed on the basis of a structural analysis of neutron powder diffraction data refined by the Rietveld analysis. Both compounds were found to crystallize in the kesterite type structure, but with a disorder within the Cu-Zn layers at z=1/4 and 3/4. The latter causes Cu(Zu) and Zn(Cu) anti-site defects, whose concentration depends on the sample growth conditions. The temperature dependent structural phase transition in CZTS was studied by in-situ high temperature diffraction experiments using high energy synchrotron X-rays. The transition from the tetragonal kesterite to the cubic sphalerite type structure is discussed by means of the interplay of increasing and decreasing cation-anion-cation bond angles in the coordination tetrahedra, reflected by the shift of the anion atomic coordinates to more ideal (x and y) or non-ideal (z) values. This striking behavior may be connected with frustrations that occur due to the triangular geometry of the coordination tetrahedra. (C) 2011 Elsevier B.V. All rights reserved.