Photo-degradation of acid orange 7 in aqueous solution was used as a probe to assess the photo-catalytic activity of titanium dioxide nanotube layers under UV irradiation. The nanotube layers were prepared by anodization of Ti foil in 0.4wt% hydrofluoric acid solution and then annealed at different temperatures between 300°C and 600°C for 1h. The nanotube layers were characterized using X-ray diffraction and scanning electron microscopy. After the 500°C anneal, the anatase and rutile mass fractions were measured to be about 55% and 45%, respectively. After annealing at 500°C, the anatase crystallite size was maximum while the size and shape of the nanotubes remain unaffected. Such TiO 2 nanotube layers showed strong photo-catalytic activity. During UV irradiation, we measured the total disappearance of the organic carbon and the formation of anion species to confirm the total mineralization of the acid orange 7.