Chemical oxidation is a popular remediation technology for contaminated soils. The injected oxidant into the contaminated soil is consumed by the contaminants as well as the soil constitutes. Decomposition of hydrogen peroxide by Mn-oxide, humic acid, pyrite and soils were examined. The decomposition by Mn-oxide and pyrite was much faster than by humic acid. Hydrogen peroxide was decomposed by redox reaction with humic acid and pyrite but was decomposed by catalytic reaction with Mn-oxide. Consumption rate of hydrogen peroxide by three natural soils (A: 1.3% organic matter, 92 mg kg-1 Mn and 1% pyrite; B: 0.15% organic matter and 928 mg kg-1; C: 0.53% organic matter and 544 mg kg-1) was mainly depending on the Mn-oxide content (A: 44.2 mg g-1 min-1; B: 1,473 mg g-1 min-1; C: 1481 mg g-1 min-1). The result of this study indicates that the concentrations of Mn-oxide and contaminant are the most important factors in the determination of oxidant demand for the chemical oxidation using hydrogen peroxide.