A moisture curable composition capable of cure to an elastomeric body comprising (a) a diluted polymer comprising (i) a silicon containing polymer of the formula X-A-X.sup.1 where X and X.sup.1 are independently selected from silyl groups which contain one or more condensable substituents per group and A is a polymeric chain having a number average molecular weight (M.sub.n) of at least 132 000; and a degree of polymerisation of at least 1800. (ii) an organic extender and/or plasticiser which diluted polymer is obtained by polymerisation in the presence of the said organic extender and/or plasticiser (b) a suitable cross-linking agent which comprises at least two groups which are reactable with the condensable groups in the diluted polymer, (c) a suitable condensation catalyst and optionally (e) one or more fillers.