The specification describes and claims a composition curable in presence of atmospheric moisture to an elastomeric body, comprising (A) the product formed by mixing a polymeric material having at least two groups bonded to silicon which are hydroxyl or hydrolysable (eg alkoxy) groups and an alkoxysilane curative and (S) a compound according to the general formula M[OR] x [OR'] y where M represents a metal having a valency of 4 selected from Group IVB of the Periodic Table and is preferably titanium, x has a value from 0 to 1, y has a value from 3 to 4 and ( x + y ) = 4, R' represents a group selected from monovalent tertiary and branched-secondary aliphatic hydrocarbon groups and R represents a monovalent linear aliphatic hydrocarbon group having 1 to 6 carbon atoms which is different from R'. Preferred compounds (B) are those in which R' represents C(R 2 R 3 R 4 ) in which each of R 2 , R 3 and R 4 represents a monovalent aliphatic chain having 1 to 6 carbon atoms, the most preferred groups R' being tertiary butyl and t-amyl groups. The compositions cure without need for the presence of a chelating agent and the preferred compositions cure quickly and do not yellow.