a process is disclosed for producing an olefinic compound from a least one compound selected from hydrocarbons and halohydrocarbons containing at least two carbon atoms and at least two hydrogen atoms . the process involves directing light from a light source through the wall of a reactor to interact with reactants comprising chlorine and said at least one compound in said reactor , thereby producing a saturated halogenated hydrocarbon having increased chlorine content by photochlorination , and dehydrohalogenating said saturated halogenated hydrocarbon produced by the photochlorination in ; and is characterized by the light directed through the reactor wall being directed through a poly polymer .