The decolorization of the reactive dye, reactive black 5 (RB5), by the photoactivated periodate (UV/IO4⁻) was investigated under ultraviolet (UV) irradiation at 254 nm, and its possible mechanism based on periodate photochemistry was discussed, in order to elucidate the main oxidants involved in the decolorization of RB5 as the reaction parameters, such as the light intensity, periodate concentration, dissolved oxygen, and solution pH, were varied, with the intermittent employment of an OH scavenger. The decolorization rate of RB5 increased linearly with increasing light intensity. The decolorization rate was found to increase at low periodate concentrations (0–5 mM), but slightly decrease at higher concentrations (>5 mM). Dissolved oxygen did not have any measurable effect on the decolorization rate of the reactive dye. No pH dependence of the decolorization rate was observed over the pH range of 1.5–10, regardless of the periodate speciation. Based on these results, the main oxidants responsible for the RB5 decolorization with UV 254 were found to be OH and IO3 (IVI), after considering the six intermediate species which are reported to be produced as a result of periodate photolysis, namely the radical species (OH, IO3, and IO4) and the non-radical species (O3, IO4⁻, and IO3⁻).