Interface potential barrier heights for ultrathin silicon oxides (15– 44 A˚) on silicon and effective electron masses in some of these oxides are evaluated. Evaluation is performed using a new technique of analyzing the charging characteristics of metal‐nitride‐oxide‐semiconductor capacitors. Oxides thicker than 36 A˚ have the same potential barrier heights as those for thick oxides, assuming the effective electron mass of the oxides is the same. However, for oxides thinner than 31 A˚, the potential barrier heights decrease and the effective electron masses increase as the oxide thickness decreases. These results suggest that oxides at least thicker than 36 A˚ can be applied to metal‐oxide‐semiconductor field‐effect transistors as gate oxides.