Abstract: Regarding to the petroleum hydrocarbon polluted sites that are remediated with H₂O₂ based ex-situ chemical oxidation, the oxidizing capacity and reactive process of catalyzed H₂O₂ were studied with different H₂O₂ mass fractions and oxidant to catalyst/stabilizer ratios. It is found that hydroxyl radical and superoxide anion are the major oxidation groups. The H₂O₂ mass fraction of 0.50%-1.00% and the oxidant to catalyst/stabilizer ratio of 75∶1-100∶1 provide higher total petroleum hydrocarbon (TPH) degradation rate. The results demonstrated that high H₂O₂ concentration and low oxidant to catalyst/stabilizer ratio may lead to rapid H₂O₂ decomposition that reduce the contact time between oxidant and contaminant and decrease the oxidizing efficiency. In addition, it is also found when H₂O₂ is under rapid decomposition, it tends to oxidize hydrocarbon molecules with less carbon number.