In this work, a new and highly active heterogeneous Fenton system based on iron metal and magnetite Fe 0 /Fe 3 O 4 composites has been prepared by controlled reduction of iron oxides. Temperature-programmed reduction experiments with H 2 showed that iron oxides, i.e. Fe 2 O 3 , FeOOH and Fe 3 O 4 , can be reduced to produce highly reactive Fe 0 /Fe 3 O 4 composites with different metal to oxide ratios as determined by Mössbauer spectroscopy and powder X-ray diffraction. Mössbauer measurements revealed that these composites are reactive towards gas phase molecules and can be oxidized rapidly by O 2 even at room temperature. The composites showed also very high activity for the Fenton chemistry, i.e. the oxidation of an organic model contaminant, the dye methylene blue, and the H 2 O 2 decomposition. The best results were obtained with the composites with 47% Fe 0 obtained by reduction of Fe 3 O 4 with H 2 at 400°C for 2h, which produced a very rapid discoloration with total organic carbon (TOC) removal of 75% after 2h reaction. Conversion electron Mössbauer spectroscopy (CEMS) measurements before and after H 2 O 2 reaction showed that Fe 3 O 4 and especially Fe 0 are oxidized during the reaction. The reaction mechanism is discussed in terms of the formation of HO radicals by a Haber–Weiss initiated by an efficient electron transfer from the composite Fe 0 /Fe 3 O 4 to H 2 O 2 . The higher activity of the composites compared to the pure Fe 0 and iron oxides has been explained by two possible effects, i.e. (i) a thermodynamically favorable electron transfer from Fe 0 to Fe 3 O 4 producing Fe 2+ magnetite active for the reaction and (ii) by the formation of very reactive small particle size Fe 0 and Fe 3 O 4 .