Band alignment at CdS/Cu2ZnSnSe4 heterojunction interface is studied by X-ray photoemission spectroscopy. The Cu2ZnSnSe4 thin films are prepared by selenization of electrodeposited Cu-Zn-Sn precursors. CdS overlayers with different thickness are sequentially grown on the Cu2ZnSnSe4 substrate by pulsed laser deposition process. Photoemission spectra are obtained before and after each growth to study the conduction and valence band offsets at the heterojunction interface. The determined conduction band offset of 0.34 eV indicates a spike-like ‘type I’ band alignment at CdS/Cu2ZnSnSe4 interface. The spike will avoid interface recombination, and it is low enough that electron could transfer from the Cu2ZnSnSe4 layer to the buffer layer which is suitable for solar cell's fabrication. Copyright © 2012 John Wiley & Sons, Ltd.