The invention provides a benzoin group-containing cyclic organopolysiloxane, which belongs to a novel class of organosilicon compounds hitherto not known or not described in any prior art literatures. The compound has at least one benzoin group of the formula ##STR1## in which Ph is a phenyl group and R.sup.1 is a hydrogen atom or an alkyl group having from 1 to 4 carbon atoms or, in particular, a methyl group, directly bonded to the silicon atom in the cyclic tri-, tetra-, penta- or hexasiloxane. The compound is useful as a sensitizer for the crosslinking reaction of a photo- or radiation-curable organopolysiloxane composition owing to the high miscibility of the inventive compounds with the organopolysiloxane composition.