Chelate formation is confirmed by a red shift of the n-->pi* absorption peak of benzoylacetone ( BzAcH) from 309 to 336 nm with its addition to a sol containing Zn2+ ions. The chelate bonds between Zn2+ and BzAc(-) are mostly maintained in the gel film prepared from the sol. Irradiation of the gel film by a Xe lamp with a cut filter (lambda(ex) > 300 nm) in the presence of H2O leads to decomposition of the chelate ring. As a result of the photolysis, ZnO-H groups, CH3CHO and other carbonyl compounds are generated with the lost of CH3 groups of the BzAc(-) ligand and H2O involved in the film. INDO/S calculations on a model complex (Zn(BzAc)(OEt)) assign the n --> pi* absorption to the electronic transition from a non-bonding molecular orbital (MO) distributed mainly at the phenyl group to an anti-bonding MO localized at the CO bonds. On the basis of these results, a photo-induced hydrolysis mechanism was presented to explain the formation of positive-type patterned ZnO films. (C) 2003 Kluwer Academic Publishers.