Mesoporous Al2O3-TiO2 nanocomposites at different Al2O3 contents (0-5 wt.%) have been synthesized through a facile sol-gel method using tetrabutyl orthotitanateand and aluminum-tri-sec-butoxide as a precursors for TiO2 and Al2O3 sources. XRD and Raman spectra confirmed that highly crystalline anatase TiO2 phase was formed at low Al2O3 content; however, the produced Al2O3-TiO2 nanocomposites are amorphous phase at 3 and 5 wt.% Al2O3. TEM images show TiO2 particles are quite uniform with 10 +/- 2 nm sizes with mesoporous structure. The surface area of mesoporous TiO2 was increased from 174 m(2)/g to 325 m(2)/g by increasing Al2O3 contents from 0% to 5%. The photocatalytic efficiencies of Al2O3-TiO2 nanocomposites were evaluated by photodegradation of herbicide imazapyr compared with UV-100 under UV illumination. The photocatalytic efficiency of 2 wt.% Al2O3-TiO2 nanocomposite is significantly 2 and 3-fold higher than that of mesoporous TiO2 and UV-100, respectively. 2 wt.% Al2O3-TiO2 is considered to be the optimum photocatalyst for the highest photonic efficiency similar to 4%. The superiority of 2 wt.% Al2O3-TiO2 nanocomposite is explained by the highly crystalline and small particles sizes with mesoporous structure. The proposed mechanism of this system and the role of amorphous Al2O3 are explained by details. (C) 2015 Elsevier B.V. All rights reserved.