The photocatalytic degradation of aqueous solutions of Acid Orange 7 in TiO2 suspensions has been investigated with the use of a solar light simulating source. The photoreaction was followed by monitoring the degradation of the dye and the formation of intermediates and final products, as functions of time of irradiation, both in solution and on the photocatalyst surface. It has been found that the dye adsorbs on TiO2 and undergoes a series of oxidation steps, which lead to decolorization and formation of a number of intermediates, mainly aromatic and aliphatic acids. These molecules are further oxidized toward compounds of progressively lower molecular weight and, eventually, to CO2 and inorganic ions, such as sulfate, nitrate and ammonium ions. A TiO2-mediated photodegradation mechanism for Acid Orange 7 is proposed on the basis of quantitative and qualitative detection of intermediate compounds.