The invention provides a composition comprising a polyolefin and a polyorganosiloxane in which at least 50 mole % of the siloxane units are D units as herein defined and which contains at least one unsaturated group, characterized in that the unsaturated group is group of the formula —X—CH═CH—R″ (I) or —X—C≡C—R″ (II), in which X represents a divalent organic linkage having an electron withdrawing effect with respect to the —CH═CH— or —C≡C— bond and/or containing an aromatic ring or a further olefinic double bond or acetylenic unsaturation, the aromatic ring or the further olefinic double bond or acetylenic unsaturation being conjugated with the olefinic unsaturation of —X—CH═CH—R″ or with the acetylenic unsaturation of —X—C≡C—R″ and R″ represents hydrogen or a group having an electron withdrawing effect or any other activation effect with respect to the —CH═CH— or —C≡C— bond.