The photo-catalytic degradation efficiency of several commercial titania powders (Degussa P-25 and Ishihara Sangyo ST-01) and of TiO2 supported on diatomite were investigated in the degradation of a reactive dye (RED-3BA) under UV-Visible light. The observed rate constant for Degussa P-25 was found to be higher than that achieved by using Ishihara Sangyo ST-01 (kobs=3.3×10−2min−1 for Degussa P-25 and 4×10−4min−1 for Ishihara Sangyo ST-01). This could be related to the amount of basic hydroxyl groups on the surface of TiO2 particles as shown in the IR spectra. TiO2 Degussa P-25 supported on diatomite was prepared by dip-coating method. The photo-catalytic activity of supported TiO2 was twice smaller than TiO2 Degussa P-25 (kp-25=3.3×10−2min−1;kp25/diatomite=1.6×10−2min−1). The higher surface areas of ST-01 and the TiO2/diatomite could not lead to a higher degradation rate and to a higher degree of mineralization but TiO2/diatomite could be separated more promptly and more easily from the solution. The effect of pH was investigated in the range 4–9. Acidic (pH = 4) medium was found to favor the adsorption and degradation rate with P-25 particles.