The effects of UV irradiation on the properties of xAl(2)O(3) .(100-x)SiO2 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 pi-pi* transition in chelate rings of beta-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 HNO3. This finding was successfully applied to the fine patterning of xAl(2)O(3) .(100-x)SiO2 gel films with x = 30 or above. The gel films were leached in ethanol containing HNO3, after UV irradiation through a mask, and heat-treated at 400 degrees C for 20 min to give patterned amorphous Al2O3-SiO2 films.