Iron oxide nanoparticles (Fe-ox NPs) have been supported on the hydroxylated surface of modified diamond nanoparticles (D3). Characterization data show that, once formed, Fe NPs are spontaneously oxidized under ambient conditions, exhibiting a good dispersion of small oxidized Fe-ox NPs (2.2 +/- 0.5 nm) on D3. It has been observed that the activity of Fe-ox/D3 as heterogeneous Fenton catalyst for phenol degradation by H2O2 can be assisted by visible light irradiation. Fe-ox/D3 exhibits a superior activity compared with analogous catalysts based on activated carbon, graphite or the benchmark Fe-ox/TiO2 photocatalyst. Fe-ox/D3 exhibits comparable activity to Ag/D3 that is one of the most active catalysts ever reported for this reaction. In addition, Fe-ox/D3 presents higher stability and recyclability than analogous Cu/D3. A minimum accumulated turnover number of 38,000 can be achieved using Fe-ox/D3 as photocatalyst. The heterogeneous photoFenton process using phenol as model pollutant and Fe-ox/D3 as catalyst under visible light irradiation can be implemented prior to an aerobic biological treatment resulting in a biodegradable effluent which lacks ecotoxicity, as determined by measurement of the biological oxygen demand. Transient absorption spectroscopy provides evidence in support of the formation upon irradiation of Fe-ox/D3 of photogenerated charge separation state attributed to electrons and holes. Electron paramagnetic resonance and selective quenching experiments indicate that hydroxyl radicals are the main reactive oxygen species generated in the photo-assisted Fenton reaction promoted by Fe-ox/D3.