Compositions curable by exposure to UV or electron beam are disclosed which contain mercapto functional organosiloxanes and organosiloxanes bearing alkenyl groups with 6 or more carbon atoms. The compositions cure upon irradiation without the emission of obnoxious odor which characterizes the analogous composition of the art containing olefinic siloxanes having only vinyl unsaturation. Also disclosed is a method of treating a substrate with the coating composition to improve the release of adhesive materials therefrom.