Nb-doped iron oxides were used as heterogeneous catalyst to oxidize organic compounds in aqueous medium containing hydrogen peroxide. XRD data reveal that the composites contain hematite (α-Fe2O3), maghemite (γ-Fe2O3) and FeNb2O6. The H2O2 pretreatment of the solid catalyst promotes important surface and structural changes of the iron oxides mainly by peroxo-niobium complexes formation, which expressively enhances the catalytic properties of the composite. Transmission electron microscopy images show that the H2O2-treatment tends also to decrease the mean particle size of grains of the composite. Nb-doped iron oxides were found to impart an important role to the solid catalyst towards H2O2 reactions. The developed composites were confirmed to have remarkable catalytic activity on the oxidation of organic substrates and can be regenerated for several successive reaction cycles for degrading polluting organic in water.