Almost fifteen percent of the total world production of dyes is lost during the dyeing process and is released in the textile effluents. Removal of colour and organics by photocatalytic degradation is emerging as an effective treatment technique. Photodegradation process occurs with the attack of organic substances by the activated oxygen species, such as hydroxyl radical and super oxide radical, generated on TiO2 particulate. The irradiation of titania with visible light did produce a photoinduced decolorization of the dye, induced by the breaking of the double-bond conjugation system of the chromophoric group. Observation of the decolorization and of the degradation of solid indigo constitutes a surprising and encouraging result for the development of self-cleaning titania-coated surfaces. In addition, we report high level computational calculations on the indigo + OH radical reaction, aiming to clarify their mechanism and kinetics. Energy profiles are calculated along all possible pathways. Electronic structure calculations have been performed with the Gaussian 09 program using the M05-2X/6-311++g(d,p) method. Arrhenius parameters, tunneling corrections, and rate constants are obtained and compared with the available experimental data. In conclusion, our theoretical and experimental results suggest that TiO2/UV photocatalysis could be a adequate method for treatment of wastewaters in textile industries.