The band offset at the CdS/Cu2ZnSnS4 heterojunction interface is studied by measuring the valence band spectra using synchrotron radiation photoemission spectroscopy. The Cu2ZnSnS4 thin films are prepared by the sulfurization of electrodeposited Cu-Zn-Sn precursors. A CdS overlayer is sequentially grown on the Cu2ZnSnS4 thin films from a chemical bath deposition process. Valence band spectra were obtained before and after each period of growth to study the electronic structure at the heterojunction interface. The valence band offset was determined to be 0.96 eV, and the conduction band offset was determined to be .0.06 eV. This means that the CdS/Cu2ZnSnS4 hetrojunction has a ‘type II’ band alignment which will cause largescale recombination at the interfaces and will not be suitable for solar cells fabrication.