An SiOx (x > 1.5) overlayer has been successfully grown on ZnS (111) crystal. Synchrotron radiation photoemission spectroscopy has been used in situ to measure the electronic structure and band lineup of the heterojunction. The valence band offset of SiOx/ZnS (111) derived from the measurements is 2.8 +/- 0.2 eV. This agrees well with the theoretical result predicted by the revised Harrison's 'Tight Binding' theory. The experimental result also explains the positive role of SiO2 layers in thin-film electroluminescence devices.