In this work, the effect of incorporation of M 2+ species, i.e. Co 2+ , Mn 2+ and Ni 2+ , into the magnetite structure to increase the reactivity towards H 2 O 2 reactions was investigated. The following magnetites Fe 3−x Mn x O 4 , Fe 3−x Co x O 4 and Fe 3−x Ni x O 4 and the iron oxides Fe 3 O 4 , γ-Fe 2 O 3 and α-Fe 2 O 3 were prepared and characterized by Mössbauer spectroscopy, XRD, BET surface area, magnetization and chemical analyses. The obtained results showed that the M 2+ species at the octahedral site in the magnetite strongly affects the reactivity towards H 2 O 2 , i.e. (i) the peroxide decomposition to O 2 and (ii) the oxidation of organic molecules, such as the dye methylene blue and chlorobenzene in aqueous medium. Experiments with maghemite, γ-Fe 2 O 3 and hematite, α-Fe 2 O 3 , showed very low activities compared to Fe 3 O 4 , suggesting that the presence of Fe 2+ in the oxide plays an important role for the activation of H 2 O 2 . The presence of Co or Mn in the magnetite structure produced a remarkable increase in the reactivity, whereas Ni inhibited the H 2 O 2 reactions. The obtained results suggest a surface initiated reaction involving M surf 2+ (Fe, Co or Mn), producing HO radicals, which can lead to two competitive reactions, i.e. the decomposition of H 2 O 2 or the oxidation of organics present in the aqueous medium. The unique effect of Co and Mn is discussed in terms of the thermodynamically favorable Co surf 3+ and Mn surf 3+ reduction by Fe magnetite 2+ regenerating the active species M 2+ .