[en] Thermal tantalum oxide with a thickness of 620 A was studied. The dc leakage resistance and high-frequency (1-MHz) resistance of a metal--tantalum-oxide--silicon capacitor were found to be on the order of 10⁸ and 1 Ω cm², respectively. The C-V behavior of the capacitor, with its initial states being carefully treated, was reproduced and observed to be dependent on the return voltage and hold time (at return point) of the measurement conditions. And only negative charges were observed to be responsible for the conduction current through tantalum oxide. A model with the considerations of the ac equivalent circuit and low-frequency leakage characteristic of tantalum oxide was proposed for these observations. Theoretical examples, with their parameters being suitably given according to the measured data, were shown, and they explained the experimental observations quite well. It is found that the measurement conditions and effect of the ac resistance of tantalum oxide on the determination of flat-band capacitance are important and should be carefully considered when one is interpreting the interface charges from C-V curves