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Ionizing radiation technology was applied as an innovative oxidation process for removing pharmaceutical contaminants from wastewater. This investigation was designed to study the degradation and mineralization of an anti-inflammatory drug diclofenac (DCF) in aqueous solutions treated by two irradiation processes in different conditions. Highest degradation efficiency reached after an optimization of the main parameters was 99% at 7 kGy for lower concentrations and 80% for higher concentrations. Moreover, the combination of irradiation and oxidizing agents could effectively enhance the radiolytic degradation of DCF. 60% and 85% of DCF (0.2 mM) was degraded at 1.5 kGy in the presence of H2O2 (5 mM) and K2S2O8 (1 mM), respectively. The ionizing process follows pseudo-first-order kinetics. The performance of this process was followed by an exhaustive analytical study and a preliminary degradation mechanism was suggested based on electron paramagnetic resonance (EPR) and LC-MS identified products. EPR spectroscopy, used to identify fragments from main intermediates formed during ionization, confirmed the formation of an imine-quinone product characterized by a g factor = 2.00362, permitting to deduce that the electrophilic hydroxyl radical •OH has reacted with C=C double bonds of aromatic ring. All results provided that this process is effective for the degradation of persistent pharmaceutical in aqueous solutions due to strong oxidative properties of generated radicals mainly hydroxyl radical. [ABSTRACT FROM AUTHOR]