Decolorization of Reactive Red 239 (RR239) using a sulfate radical (SO₄^.-)-based advanced oxidation process was investigated in a batch photoreactor. SO₄^.- was generated in situ through activation of peroxydisulfate under UV-C illumination. Effect of initial pH (3-9), initial dye concentration (20-50 mg dm^-3), S₂O₈^2- dosage (0-3 mmol dm^-3) and lamp power (0-16 W) were explored. It was found that initial pH of RR239 solution had no considerable effect on decolorization efficiency. Experimental results demonstrated that decolorization efficiency enhanced with increasing S₂O₈^2- dosage and lamp power and decreasing initial dye concentration. Quenching experiments were performed with alcohols to determine the dominant radical. Under the conditions tested in this study, 98% and 70% decolorization and aromatic degradation efficiencies were obtained within 120 min of irradiation time, respectively. Decolorization and aromatic degradation efficiencies of RR239 in PS/UV-C and PMS/UV-C systems were evaluated in terms of electrical energy consumption per order by figure of merit approach.