Here, several γ-Fe 2 O 3 /SiO 2 nanocomposite microspheres were synthesized, characterized and tested as magnetically separable heterogeneous catalysts in an advanced oxidation process. A strong relation between the structure and the activity of these materials was evidenced. Two parameters, including the porosity of the silica support and the doping of the material with Fe 3+ ions in addition to the γ-Fe 2 O 3 nanoparticles, were investigated. The structure of the catalytic materials was determined by the use of different methods, in particular SEM, TEM, XRD, N 2 physisorption, magnetometry, UV-Vis spectroscopy and XPS. The results demonstrated that the irregular microporosity of the silica framework can be replaced by an ordered mesoporosity, adding a second surfactant during the synthesis. For the mesoporous material, it was observed that the additional Fe 3+ are dispersed in the mesoporous silica framework as individual cations and small clusters of Fe(III) species. The materials were tested as heterogeneous Fenton catalysts on two model aqueous pollutants, methyl orange (MO) and reactive black 5 (RB5). It was observed that the oxidation and mineralization rates of the two dyes is significantly enhanced when a doped catalytic material is used. This effect is particularly strong with the mesoporous silica support. In addition, the doped mesoporous catalyst retained its activity for a large range of conditions. These improvements were attributed to the strong activity of the additional Fe(III) species, although the presence of the mesoporosity also plays a significant role.