The photocatalytic degradation mechanism of an azo dye, C.I. Reactive Black 5 (RB5), has been investigated in an aqueous suspension of SrTiO3/CeO2 composite under 250 W UV irradiation at pH 12.0. The process was studied by monitoring the change in RB5 concentration and the intermediate products employing UV-visible spectrophotometry, ion chromatography (IC), and gas chromatography/ mass spectrometry (GC/MS) techniques and depletion in total organic carbon (TOC) content as a function of irradiation time. The adsorption peaks at wavelengths of 600, 312, and 254 nm were identified as the chromophore structure, and the naphthalene and benzene components of RB5, respectively. Little influence of iodide ion, tert-butyl alcohol, fluoride ion, or persulfate ion as h(vb)+, *OH, or e(cb)- scavengers on the decolorization proved that the decolorization of RB5 primarily proceeded by photolysis and/or O2*- in the bulk solution. After the decolorization, the process could shift progressively from the bulk solution to the surface of the catalysts and cleavage of the naphthalene and benzene rings was mainly attributed to the h(vb)+ pathway and *OH(ads) reactions, which was further verified by the effect of scavengers.