There are prepared compounds of the formula [Rn1(R2O)3-n-Si-Alk-S-]xZ(I) where Z is 3BOND PO, 3BOND PS, 3BOND P, =PR, -P(R)2, -P(OR)2, =P(OR), -PO(OR)2, -PS(OR)2, =PO(OR) or =PS(OR) where R is an alkyl group of 1 to 5 carbon atoms, phenyl or o-, m- or p-nitrophenyl; Alk is an alkylene group with 2 to 4 carbon atoms; R1 is an alkyl group having 1 to 5 carbon atoms, benzyl or phenyl; R2 is an alkyl group having 1 to 5 carbon atoms, a cycloalkyl group having 5 to 8 carbon atoms, phenyl, benzyl or 2-methoxyethyl, each R1 and R2 can have the same or different meanings; n is 0, 1 or 2; and, x is 1, 2 or 3, being the residual valence of Z. The compounds are useful as reinforcing agents in vulcanizable rubber mixtures which contain a silicate filler and optionally carbon black.