Abstract The effects of UV irradiation on the properties of xAl 2O 3·(100−x)SiO 2 gel films, which were obtained from aluminum butoxide chemically modified with benzoylacetone and partly hydrolyzed silicon ethoxide, have been studied. These gel films showed an optical absorption band at around 325 nm characteristic of the π–π∗ transition in chelate rings of β-diketonate ligands. The irradiation of the gel films with UV light of 365 nm dissociated the chelate ring and simultaneously decreased the solubility of the gel films in ethanol containing HNO 3. This finding was successfully applied to the fine patterning of xAl 2O 3·(100−x)SiO 2 gel films with x = 30 or above. The gel films were leached in ethanol containing HNO 3, after UV irradiation through a mask, and heat-treated at 400°C for 20 min to give patterned amorphous Al 2O 3–SiO 2 films.