The efficiencies of the advanced oxidation processes (AOP) viz. photocatalysis, ozonolysis and gamma radiolysis in the absence and presence of potassium persulfate (K₂S₂O₈) were systematically investigated for the treatment of simulated textile dye waste water (STDWW) containing Reactive Red 120. The oxygen-equivalent chemical-oxidation capacities of photocatalysis, ozonolysis and gamma radiolysis in the absence and presence of K₂S₂O₈ for 16% mineralization of STDWW were calculated as 4.02, 16.19, 0.13, 0.05 kg equiv. O₂ m⁻³, respectively. The gamma radiolysis in the presence of K₂S₂O₈ showed the maximum extent of mineralization among these three AOPs. The pulse radiolysis studies revealed that the favourable reaction of SO₄˙⁻ with SDBS (the most robust organic component of STDWW) producing benzyl and hydroxycyclohexadienyl type of radicals caused the enhancement in the extent of mineralization of STDWW during gamma radiolysis in the presence of K₂S₂O₈. The COD of the STDWW was brought down to 1558 ppm from 3128 ppm by gamma radiolysis at 50 kGy dose in the presence of K₂S₂O₈; though that index could not meet the allowed discharge limit (COD ≤ 250 ppm) of industrial effluent in the main water stream. Conversely, on replacing organic acid (CH₃COOH) by inorganic acid (H₂SO₄) in the pH adjustment step, COD of STDWW was brought down to 245 ppm by gamma radiolysis at 60 kGy in the presence of K₂S₂O₈. This paper recommends using H₂SO₄ in place of CH₃COOH in the pH adjustment step followed by the gamma radiolysis of STDWW in the presence of K₂S₂O₈ for an effective effluent treatment.