Polychloro compounds were added to olefins in the presence of semiconductor particles (Ti02 or CdS) under photoirradiation. The photoreaction without semiconductors afforded no adducts at all; however, loading of Ti02 with Ag, Cu, or Fe30r enhanced the yield of the photoaddition reaction even further. Semiconductors operate as catalysts for the one- electron injection into the polychloro compounds. The relative adsorptions of the ...