PURPOSE: To provide an organosiloxane compsn. being a multi-division type compsn. composed of a specific liquid diorganopolysiloxane or organohydrogensiloxane and forming an elastomer requiring no post-curing and excellent in compression permanet set and suitable for a diaphragm.

CONSTITUTION: A liquid diorganopolysiloxane (A) containing two or more silicon bonded vinyl groups per one molecule, an organohydrogensiloxane (B) compatible with the component (A) and containing two or more silicon bonded hydrogen atoms per one molecule contained in an amt. sufficient to cure a compsn. and a platinum-containing hydrosilylated catalyst (C) in an amt. sufficient to accelerate the curing of the compsn. within a temp. range from the arnbient temp. to 250°C and 10-60 wt.% of a reinforcing silica filler (D) are compounded and a vinyl org. peroxide is further compounded with the above mentioned compsn. (E) in an amt. of 0.2 wt.%. of E in a multi-division type compsn., the components B, C are present in a separately divided ratio.