The photocatalytic degradation of the anthraquinonic dye Acid Blue 80 in aqueous solutions containing TiO2 dispersions has been investigated. The process has been monitored by following either the disappearance of the dye (via HPLC) and the formation of its end-products (via IC, GC, and TOC analysis). Although a relatively fast decolorization of the solutions has been observed, the mineralization is slower, and the presence of residual organic compounds was evidenced even after long term irradiation, confirming the relevant stability of anthraquinone derivatives. The identification of various unstable intermediates formed after low irradiation times was performed by HPLC-MS, allowing us to give insight into the early steps of the degradation process which mainly involve C-N bonds breaking and substrate hydroxylation. Complete and relatively fast mineralization of the substrate was achieved by irradiating the semiconductor dispersions in the presence of added K2S2O8.