SiOx(x>1.5)overlayers have beenin situgrown on ZnS (111) and CdTe (111) substrates. Synchrotron radiation photoemission spectroscopy has been used to measure the electronic structures and band lineups of these two heterojunctions. The valence band offsets ofSiOx/ZnS(111)andSiOx/CdTe(111)derived from the measurements are 2.8±0.2 and 4.7±0.2 eV, respectively. Harrison’s “tight binding” theory is extended into the theoretical estimation of the band lineups ofSiO2related heterojunctions. The agreement between the experimental and theoretical results is good. Our result also explains the positive role ofSiO2layers in ZnS-based thin film electroluminescence devices.