Synchrotron radiation photoelectron spectroscopy is used to measure the valence-band offsets for Si/ZnS(111) and (100) heterojunctions. The valence band discontinuities obtained from the measurements are both (1.9±0.1)eV for Si/ZnS(111) and Si/ZnS(100) interfaces,which is in excellent agreement with the theoretical predictions but considerably different from the experimental- result of ZnS/Si(111) reported by Maierhofer et al. This suggests that the commutativity rule of- band offset may not be valid for Si/ZnS polar interface and the reason is discussed.