A one or two part organopolysiloxane composition capable of cure to an elastomeric body, the composition comprising a polymer having a polymer backbone selected from an organopolysiloxane backbone and a telechelic backbone, which polymer has not less than two reactable silicon-bonded groups selected from alkenyl group, condensable groups, silyl-hydride groups, 5 to 50% by weight of the composition of an extender; a suitable cure catalyst; and optionally a siloxane and/or silane cross-linker having at least two groups per molecule which are reactable with the reactable groups in (a) and/or one or more fillers. The extender is selected from an alicyclic ester having at least two ester groups, and a boiling point of at least 180° C. or a mixture thereof having a boiling point range commencing at 180° C. or above; each ester group being the same or different and comprising from 2 to 30 carbon atoms.