Abstract: Composition which comprises a mixture of (A) a polydiorganosiloxane having a viscosity of from 200 to 100,000 cP at 25.degree. C. and having at least two silicon-bonded vinyl groups, (B) an organohydrogensiloxane, (C) a platinum catalyst, (D) a substance which inhibits the reaction between (A) and (B) in the presence of (C) e.g. an acetylenic alcohol, and (E) a blowing agent selected from p,p'-oxybis(benzenesulphonylhydrazide) and dinitrosopentamethylene tetramine.The composition can be converted by heat into an elastomeric sponge and is useful for the production of articles such as seat cushions, heat and sound insulation and shock absorbers.