Abstract: This invention relates to the use of silyl terminated organic polymers in phenylorganosiloxane based silicone sealant formulations containing silyl terminated organic polymers are described. Subsequent to cure the compositions, provide sealants exhibiting superior mechanical properties, particularly with respect to elongation, tensile strength and adhesion on glass. The composition comprises a phenylorganosiloxane having terminal groups selected from —OH or hydrolysable groups and unsaturated groups. Typically the phenylorganosiloxane has a viscosity of at least 10000 mPa.s at 25° C. Other ingredients include either: (i) one or more organic polymers having terminal and/or pendent silyl groups containing —OH functional groups or hydrolysable functional groups, or (ii) one or more organic polymers having terminal and/or pendent silyl groups containing one or more unsaturated groups, selected in accordance with the terminal groups of (a) as well as fillers, cross-linkers and catalyst.