A process for preparing cross-linked, modified organopolysiloxanes which comprises polymerizing at least one monomer having at least one aliphatic multiple bond in the presence of a free radical initiator and at least one cross-linked organopolysiloxane containing at least 40 mol percent of monoorganosiloxane units. When the organopolysiloxane is not crosslinked, then the polymerization of the monomer having at least one aliphatic multiple bond is conducted in the presence of an organopolysiloxane having at least 40 mol percent mono-organosiloxane units, and a free radical initiator and a catalyst which promotes cross-linking of the organopolysiloxane.