One promising candidate considered for solar cell absorber layers is Cu2ZnSnS4 (CZTS). Transmission electron microscopy (TEM) investigations of such solar cells to date are scarce. We present microanalysis results on our fully processed CZTS solar cells based on absorber layers deposited by reactive sputtering of a precursor layer followed by a short anneal. The initially small grain size for precursor layers increases rapidly due to annealing, typically spanning the entire absorber layer thickness. Energy dispersive X-ray spectroscopy in a TEM clearly reveals the formation of secondary compounds containing Zn-, Cu-or Sn-sulfides located at the Mo/CZTS back contact interface after annealing. Simultaneously a MoS2 layer is formed at the back contact. The extent to which secondary compounds and MoS2 form scales with annealing time, indicating that Mo is not stable when in contact with CZTS. Understanding the chemical reactions at the back contact is considered to be essential to limit the secondary phase formation during annealing. (C) 2012 Elsevier B.V. All rights reserved.