In this work, photocatalytic degradation of an anthraquinone dye, C.I. Acid Green 25 (AG25), under UV light irradiation using TiO2 nanoparticles in a rectangular photoreactor was studied. The investigated TiO2 was Millennium PC-500 (crystallites mean size 8 nm and surface area of 320.76 m(2)/g) immobilized on glass plates. Response surface methodology (RSM) was employed to assess individual and interactive effects of the four main independent parameters (initial dye concentration, UV light intensity, flow rate and reaction time) on the decolorization efficiency. Central composite design was used for optimization of UV/TiO2 process. Predicted values of decolorization efficiency were found to be in good agreement with experimental values (R-2 = 0.9308 and Adj-R-2 = 0.8702). Optimization results showed that maximum decolorization efficiency was achieved at the optimum conditions: initial dye concentration 10 mg/L, UV light intensity 47.2 W/m(2), flow rate 100 mL/min and reaction time 200 min. Photocatalytic mineralization of AG25 was monitored by total organic carbon (TOC). The degradation pathway of AG25 was proposed based on the identified compounds by GC-MS technique. (C) 2010 Elsevier B.V. All rights reserved.