The aqueous phase oxidation of m-xylene and tetrachloroethylene has been studied in a batch reactor over a temperature and pressure range of 523 to 563 K and 10.3 to 17.2 MPa, respectively, in the presence of a well-characterized soil. Oxidation rates have been compared to previously measured rates in the absence of soil. The soil has been found to have a serious inhibiting effect on m-xylene oxidation, but to have virtually no effect on the rate of tetrachloroethylene disappearance. The m-xylene inhibition is interpreted by postulating that the soil serves as a free radical scavenger, thus preventing the concentration of free radicals in solution from reaching the level required for the chain mechanism to become self-propagating. An alternate and substantially slower mechanism then becomes responsible for m-xylene oxidation in the presence of soil. In contrast, soil-free tetrachloroethylene oxidation is not thought to proceed by a free radical mechanism. Hence, the addition of the soil has no effect on its oxidation rate.