Epitaxial Cu 2 ZnSnSe 4 (CZTSe) thin films have been grown via high temperature coevaporation on GaAs(001). Electron backscattering diffraction confirms epitaxy in a wide compositional range. Different secondary phases are present in the epitaxial layer. The main secondary phases are Cu 2 SnSe 3 and ZnSe which grow epitaxially on top of the CZTSe. Transmission electron microscopy measurements show that the epitaxial CZTSe grows predominantly parallel to the c-direction. Epitaxial CZTSe solar cells with a maximum power conversion efficiency of 2.1%, an open-circuit voltage of 223 mV and a current density of 16 mA/cm 2 are presented.