Abstract: A composition which is curable in the presence of moisture to an elastomer having improved adhesion to certain substrates, especially concrete. The composition comprises the product obtained by mixing together (A) a polydiorganosiloxane having terminal silanol groups, (B) an alkoxysilane R.sub.a Si(OR').sub.4-a wherein R represents a monovalent hydrocarbon or halogenated hydrocarbon group having 1-6 carbon atoms, R' is a C.sub.1 to C.sub.4 alkyl group and a is 0 or 1, (C) an organic titanium catalyst, (D) a reinforcing or semi-reinforcing filler and (E) a polydiorganosiloxane gum having a plasticity number (ASTM Test D-926) in the range from 100 to 200.The compositions are particularly suitable for use as sealants for building structures.