The synthesis, characterization and photoactivity for the MTBE decomposition on Au/TiO2-Al2O3 and AU/TiO2 sol-gel synthesized photocatalysts are reported. The semiconductor supports TiO2, and TiO2-Al2O3 were prepared by gelling titanium and aluminum alkoxides, whereas gold nanoparticles were prepared by the deposition-precipitation method with urea. Reference Au-supported photocatalysts were also prepared using the incipient impregnation method. Nitrogen adsorption as well as XRD, UV-vis and STEM-EDAX spectroscopies was used to characterize the solids. A shift of the band gap to the visible region was observed in gold-supported solids. The E-g band of the supports was shifted to the lower energy region for gold-supported catalysts. UV-vis characterization showed a gold plasmon surface resonance band (similar to 560 nm) in catalysts showing particles >7.0 nm. It is showed that the photocatalytic decomposition of MTBE in water (500 ppm) carried out with visible-light source (; 495 nm) strongly depend of the Au particle size (6.4-25 nm). The catalysts with gold particle size <= 7.5 nm are the most active. It is proposed that in the catalyst with gold particle 7.0 nm the Au delta+ electrodeficiency is the responsible of the highest activity. Meanwhile, in catalysts with gold particles >7.0 nm the plasmon surface resonance band plays an important role in the photoactivity behavior. (C) 2007 Published by Elsevier B.V.