The objectives of this study are to clarify the degradation mechanism of chemical substances using wet peroxide oxidation (WPO) under mild condition (150 degrees C) and to confirm the removal of polychlorinated biphenyls (PCBs) in soil using this oxidation process. Acetic and oxalic acids were mineralized using WPO. TOC removal rate of acetic acid was highest in the solution of pH 2.5. However, TOC removal rate was decreased with the increase in pH and TOC were hardly removed in the solution of pH 7 and 10. The decomposition rate of isobutyric acid by WPO decreased in the presence of radical scavenger (t-BuOH). The results suggested that the decomposition of chemical substances using WPO proceeded by hydroxyl radical (OH radical). PCBs in soil were also decomposed by performing WPO at 150 degrees C.