Abstract: The radical intermediates and reaction mechanism in the photooxidation of cis-and trans-stilbenes on the CdS colloid surface as well as in ethyl alcohol solution have been studied by spin-trapping ESR method. Results indicated that superoxide anion radical O₂^- and stilbene cation radical were gellerated in photooxidation processes, and atomic H were also formed on the surface of CdS colloid particles; the hydroxyl free radical played an important role in the formation of atomic H; ESR signal of O₂^- radical was observed in the presence of methyl viologen; OOH radical was produced in the presence of small amount of water; OH and carbon-centered radicals were formed in the presence of rather large amount of water; reaction rate was very slow for direct photooxidation processes. Reaction mechanisms for different processes were discussed.

Key words: Stilbene, Cadmium sulfide, Colloid, ESR, Photoreaction