In the oxidation reactions of hydroquinone under a supercritical condition (temperature of 683 K and pressure of 24.5 MPa), the conversion was found to become saturated despite the very fast initial reaction. This behavior was quite different from that under a subcritical condition (temperature of 633 K and pressure of 24.5 MPa). Under both conditions, p-benzoquinone was found to be an important intermediate. The yield of CO2 was very small, which indicates a strong inhibition effect of hydroquinone and/or its derivatives. These inhibition phenomena should be taken into account very carefully in the application of supercritical water oxidation for treating waste organic materials where a complete decomposition is very important.