Abstract Fenton reaction has been often used to treat industrial wastewater or landfill leachate. However, most mechanistic research into the Fenton reaction has been confined to low concentration conditions (usually the concentration of iron is less than 1 mM). These conditions are removed from the circumstances of real application. This is especially true in the treatment of landfill leachate in Korea. Therefore, we investigated the characteristics of the Fenton system using high concentrations of iron (mostly [Fe2+] = 1 approximately 10 mM, [H2O2] = 5 mM, [Organic (t-BuOH or methanol)] = 0 or 30 mM) and interpreted the results from the known reaction mechanisms of the Fenton system. As a result, the use of high ferrous ion (> or = 1 mM) is believed to be appropriate for producing large quantities of OH. within a short period of time, causing fast consumption of hydrogen peroxide. However, OH. scavenging by the ferrous ion, the changes of oxidation products due to the oxygen depleted conditions, and the precipitation effect of ferric ion must be considered for the successful application of Fenton reaction. On the other hand, in low ferrous ion (<1 mM), it is important to utilize the redox cycles of iron in an effective manner.