The best CZTS solar cell so far was produced by co-sputtering continued with vapour phase sulfurization method. Efficiencies of up to 5.74% were reached by Katagiri et al. The one step electrochemical deposition of copper, zinc, tin and subsequent sulfurization is an alternative fabrication technique for the production of Cu2ZnSnS4 based thin film solar cells. A kesterite based solar cell (size 0.5 cm(2)) with a conversion efficiency of 3.4% (AM1.5) was produced by vapour phase sulfurization of co-electroplated Cu-Zn-Sn films. We report on results of in-situ X-ray diffraction (XRD) experiments during crystallisation of kesterite thin films from electrochemically co-deposited metal films. The kesterite crystallisation is completed by the solid state reaction of Cu2SnS3 and ZnS. The measurements show two different reaction paths depending on the metal ratios in the as deposited films. In copper-rich metal films Cu3Sn and CuZn were found after electrodeposition. In copper-poor or near stoichiometric precursors additional Cu6Sn5 and Sn phases were detected. The formation mechanism of Cu2SnS3 involves the binary sulphicles Cu2 -xS and SnS2 in the absence of the binary precursor phase Cu6Sn5. The presence of Cu6Sn5 leads to a preferred formation of Cu2SnS3 Via the reaction educts Cu2-xS and SnS2 in the presence of a SnS2(Cu4SnS6) melt. The melt phase may be advantageous in crystallising the kesterite, leading to enhanced grain growth in the presence of a liquid phase. (C) 2008 Elsevier B.V. All rights reserved.