TiO2 photocatalysts hybridized with ZSM-5 were prepared by the impregnation of (NH4)(2)[TiO(C2O4)(2)] aqueous solution. This is one of the most ideal green processes in establishing TiO2 photocatalysts because of an organic solvent-free preparation method. The highly hydrophobic H-ZSM-5 with low Al2O3 content was effective as an adsorbent for acetaldehyde molecules. These results suggest the close relationship between the hydrophobicity of the zeolite and the high photocatalytic reactivity of TiO2 prepared on zeolites compared with unloaded TiO2 catalysts. However, acetaldehyde molecules were strongly trapped on the Na+ sites of ZSM-5 and their efficient diffusion onto the TiO2 sites was inhibited, resulting in low photocatalytic reactivity. The photocatalytic oxidation of acetaldehyde over TiO2/ZSM-5 was improved in the coexistence of H2O vapor. The role of the H2O molecules was estimated to be the efficient formation of OH radicals and the prevention of strong adsorption of acetaldehyde on the TiO2 sites. (c) 2006 Elsevier Inc. All rights reserved.