SiOx(x>1.5) overlayers have been in situ grown 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 of SiOx/ZnS(111) and SiOx/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 of SiO2 related heterojunctions. The agreement between the experimental and theoretical results is good. Our result also explains the positive role of SiO2 layers in ZnS-based thin film electroluminescence devices.