The epitaxial layers of ZnSe have been grown by photo-assisted metalorganic-vapor-phase epitaxy with the use of dimethylzinc and dimethylselenide as precursors, and the growth mechanism has been investigated. The decomposition of precursors was observed by in situ measurements of ultraviolet photoabsorption spectra. The results show that dimethylzinc is decomposed by reaction with hydrogen molecule and dimethylselenide is decomposed by photocatalysis on the surface of ZnSe. New reaction models are proposed.