Four silicon samples cut from a 10 ohm-cm n-type silicon polished wafer were passivated by four different oxidation processes: thermal, TCE, anodic and dc plasma. C-V characteristics of MOS capacitors fabricated with these samples and the effect of bias temperatures stress were studied. The Si-SiO//2 interface state density and mobile charge density are found to be the lowest for samples subject to anodic oxidation and the largest for samples subject to thermal oxidation. The possible causes for the observations are discussed.