Polyorganosiloxanes having an average of more than two siloxane repeat units, at least one of which repeat units includes a photoinitiating group bound to the silicon atom thereof, the photoinitiating group having the formula ##STR1## where R.sup.1 and R.sup.2 are the same or different alkyl groups or R.sup.1 and R.sup.2 are alkylene groups linked to each other so that R.sup.1 and R.sup.2 together with the carbon atom to which both are attached comprise a cycloalkyl group, and R.sup.3 is H or hydrocarbyl.Compositions of these polyorganosiloxanes and silicones or organic monomers with free radical curable groups are readily cured by irradiation with UV light.