Visible light has shown to greatly enhance the activity of iron-doped semiconductor catalyst in a Fenton reaction system, however, the mechanism for this activity enhancement remains unclear. This work attempts to reveal the origin of this visible light driven activity enhancement by studying the iron doped semiconductor catalyst Fe/gC(3)N(4) in both photo-Fenton and dark Fenton reactions. Through analysis of XPS spectra of catalyst, it is found that Fe2+/Fe3+ ratio for Fe/g-C3N4 catalyst remains unchanged and sustained during the photo-Fenton reaction cycles. This is in direct contrast to the progressively decreased Fe2+/Fe3+ ratio observed in the dark Fenton reaction. It is clear that under visible light, Fe3+ is able to capture the photo-generated electrons from the semiconductor and revert back to Fe2+, thus sustain the catalytic activity during the photo-Fenton reaction. This new mechanistic insight will guide the future design and optimization of high performance catalysts by immobilizing transition metal ions in semiconductors for advanced oxidation processes.