Modeling and optimization of decolorization of C.I. Reactive Orange 16 (RO16) via SO4 center dot- by response surface methodology (RSM) have been investigated. SO4 center dot- was generated in situ by activation of persulfate via UV irradiation. Planning of experimental runs was made by central composite design technique of RSM. A 2(4) full factorial design was applied to evaluate the interactive effects of process variables of Na2S2O8 dosage (X-1, g L-1), initial dye concentration (X-2, mg L-1), temperature (X-3, degrees C) and time (X-4, min) on decolorization efficiency considered as response (Upsilon). A second-order model for decolorization of RO16 was obtained and the experimental results fitted the model predictions well. The optimum reaction conditions to obtain maximum decolorization efficiency was found as Na2S2O8 dosage of 1.60 g L-1, dye concentration of 31 ppm, temperature of 33 degrees C and time of 66 min. Quenching studies showed that the main reactive radical was SO4 center dot-. The second-order reaction rate constant between SO4 center dot- and RO16 was found to be 1.36 x 10(9) M-1 s(-1).