Several dark‐current and photocurrent techniques have been used to determine the nature of high dark conductivity observed in oxides grown from polycrystalline silicon. Photocurrent measurements on polycrystalline Si‐SiO2‐Al MOS structures give idential interface energy barrier heights to those on single‐crystal Si‐SiO2‐Al MOS structures, and do not show the presence of any measurable oxide charge. Dark‐current measurements on polycrystalline Si MOS structures oxidized to varying degrees show an abrupt conductivity decrease when the polycrystalline Si is completely oxidized to the underlying single‐crystal Si substrate. It is concluded from these experiments that the high dark conductivity observed is an interface phenomenon that is due to localized field enhancement near the injecting contact. This field enhancement, not being due to positive oxide charge, is speculated to be caused by surface asperities.