The photooxidation of aniline to azobenzene in ethanol on US with natural sunlight and UV irradiation (365 nm) was studied as a function of [aniline], catalyst loading, airflow rate, solvent composition, etc. The metal sulfide shows sustainable catalytic activity. The product formation is larger with illumination at 254 nm than at 365 nm. Electron donors like triphenylphosphine, diphenylamine and hydroquinone favor the photocatalysis. Singlet oxygen quencher, azide ion does not inhibit the oxidation. The photocatalysis occurs in protic and aprotic solvents. The mechanism of photocatalyzed oxidation is discussed and the product formation examined using a kinetic equation. (c) 2005 Elsevier Ltd. All rights reserved.