The photoformation of azobenzene by the oxidation of aniline on ZnO in ethanol with natural sunlight and UV irradiation (365 nm) was studied as a function of [aniline] catalyst loading, airflow rate, solvent composition, etc. The photocatalyst exhibits sustainable catalytic activity. The product formation is larger with illumination at 254 nm than at 365 nm. Electron donors like triphenylphosphine, diphenylamine and hydroquinone enhance the photocatalysis. Singlet oxygen quencher azide ion does not suppress the catalysis. The photocatalysis occurs in protic and aprotic solvents. The mechanism of photocatalysis is discussed and the product formation analyzed using a kinetic model. (C) 2005 Elsevier B.V. All rights reserved.