The epitaxial layers of ZnSe have been grown by photo-assisted metalorganic che㎜cal vapor deposi tion with the use of dimethylzinc and dimethylselenide as sourse materials and the growth mechanism has been investigated. The decomposition of source materiais were observed by in site measurements of ultraviolet photo-absorption spectra. The results show that dimethylzinc is decomposed by the reaction with hydrogen molecule and dimethylselenide is decomposed by photo-sensitization effect on the surface of ZnSe. New reaction models are proposed.