BACKGROUND: Particles are often too small to be separated from a reaction system and recycled, especially in waste-water treatment via a catalytic ozonation process. Therefore, the objective of this study was to prepare a magnetic catalyst (SiO2/Fe3O4) that can be recycled by using an external magnetic field. The characteristics of the magnetic catalyst and kinetics of decomposition of Reactive Black 5 (RB5) using a magnetic catalyst/H2O2/O-3 have been investigated. Magnetic catalysts (SiO2/Fe3O4) were characterized by X-ray diffraction (XRD), vibrating sample magnetometry (VSM) and Fourier transform infrared (FTIR) spectroscopy. RESULTS: In the case of the decomposition of RB5 by the SiO2/Fe3O4/H2O2/O-3 process, the contribution of the destruction by the indirect oxidation of OH. caused by the chain reaction of H2O, OH-, and H2O2 was found to be larger than that of the direct attack of O-3. The kinetic behaviour of the reacting species during the decomposition of RB5 by catalytic ozonation could be well modelled under various experimental conditions. CONCLUSIONS: The paramagnetic behaviour of the prepared SiO2/Fe3O4 led to the formation of the magnetic catalyst SiO2/Fe3O4, which could be separated more easily by the application of a magnetic field. More than 90% of the magnetic catalyst was recovered and easily redispersed in a solution for reuse by mixing with a flat-bladed turbine. (C) 2010 Society of Chemical Industry