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Porous platinum-ion-doped TiO2 (Pt-TiO2) with BET surface area higher than 200 m2 g−1 was prepared by using water as solvent and conducting dialysis in a sol–gel method and demonstrated to have superior photocatalytic activity for the photodegradation of 4-chlorophenol (4-CP) under both ultraviolet and visible light (VL) irradiation. During dialysis, 76.5% of Cl− was removed and mesopores were formed in Pt-TiO2. Measurements of pore-size distribution indicated a peak at ca. 3 nm after the calcination of Pt-TiO2 at 200 °C. Optimal conditions to prepare Pt-TiO2 acting under VL were 0.5 at.% as the Pt-doping content and 200 °C as the calcination temperature in the range of 200–500 °C. Kinetic studies were performed to clarify the effect of the amount of Pt-TiO2, initial concentration of 4-CP, intensity and wavelength of the incident light, pH and temperature on the reaction rate and to obtain the rate law under VL irradiation. Formation of benzoquinone and hydroquinone as intermediates was observed. They were further photodegraded on Pt-TiO2 and 90.7% of 4-CP was completely mineralized at the VL irradiation of 390 min.