The decolorization of the azo dye Reactive Black 5 in aqueous solution has been studied using UV/TiO2 and photo-Fenton processes. The effects of operating parameters, such as pH, H2O2 dosage, TiO2 dosage, Fe2+ dosage and the initial dye concentration, were investigated. The optimal conditions for each of the processes were obtained. The kinetics data in both the cases fitted well to the first-order equation. A comparison of the results under the optimal conditions showed that the azo dye Reactive Black 5 was found to undergo substantial and rapid decoloration by both the method. However, the decoloraztion of the azo dye by the photo-Fenton processes was much more efficient.