The production of the reactive oxidant (i.e., hydroxyl radical) by the Fenton-like reaction (Fe(III)/H₂O₂) was greatly improved by the addition of carbon materials such as powdered activated carbon (PAC) and carbon nanotubes (CNT). The presence of carbon materials enhanced the oxidative conversion of methanol into formaldehyde by the Fe(III)/H₂O₂ system. CNT exhibited a slightly higher activity than PAC. In the presence of the carbon materials, the reduction of Fe(III) to Fe(II) was accelerated while the decomposition rate of H₂O₂ increased, which appears to be responsible for the enhanced oxidant production. The reducing power of carbon materials was also shown by the reductive conversion of polyoxomolybdate. Increasing the addition of carbon materials exhibited saturated (or slightly decreased) oxidant yields, while decreasing the H₂O₂ utilization efficiency. The enhanced oxidant production by carbon materials was more pronounced in the absence of dissolved oxygen, which was explained by the reactions of organo-radicals produced by methanol oxidation.