Cu2ZnSnSe4 (CZTSe) thin films were synthesized by a two-step process involving co-sputtering metallic precursor followed by reactive annealing. Two different techniques of annealing were used, i.e. graphite box and three-zone furnace. Very important differences were observed between the two techniques in terms of Se diffusion through the precursor and reaction path leading to CZTSe formation. Thermodynamic analysis was used in order to explain the reaction mechanism and kinetics of CZTSe formation during the annealing step. The films exhibit large grains with high crystalline quality and solar cells with conversion efficiency of 7.1% were synthesized. (C) 2014 Elsevier B.V. All rights reserved.