In recent years pharmaceuticals have become an environmental problem due to their presence in wastewater, surface water and even drinking water sources. A large number of medications are used daily, leaving residual waste in wastewater, and the elimination processes by water treatment plants are not efficient in removing these compounds. The drug that is most highly-detected in the aquatic environment is the analgesic diclofenac (DCF). The main metabolite generated by the metabolization of DCF in humans is 4′-hydroxydiclofenac. Hydroxylated diclofenac has been identified through DCF photodegradation processes. This structure undergoes additional hydroxylation in the benzene ring containing the chlorine atoms, causing their loss and subsequent hydroxylation and, finally, resulting in the cleavage of the C-N bond. In the present work, 23 degradation byproducts of 1 mg L-1 of DCF under 0.20 mg L-1 of Fe2+ and 1 mg L-1 of H2O2 at an irradiance of 425 W m-2 were identified. Due to its complex structure and the non-selective nature of the hydroxyl radical (OH•), several degradation routes are proposed for DCF.