A process for the preparation of a thiocarboxylate silane which comprises reacting (a) an aqueous solution of a salt of thiocarboxylic acid represented by the formula: R-C(=O)-S (-) M (+) wherein R is a branched or straight-chain alkyl containing up to 30 carbon atoms; and M (+) is sodium ion, potassium ion, or ammonium ion with (b) a silane represented by the formula: L f G 2 (-SiZ 3) c wherein G 2 is -CH 2 CH 2 CH 2 -; each occurrence of L is independently chloro group or bromo group; each occurrence of Z is independently RO- or -R, wherein each R is independently defined as above, with the proviso that at least one Z is not -R; c is 1; and f is 1; wherein the salt of the thiocarboxylic acid (a) is provided as an aqueous solution and the reaction is carried out in the presence of a catalytically effective amount of an alkylguanidinium salt phase transfer catalyst having the formula: wherein each of R 1, R 2, R 3, R 4 and R 5 is a C 1 to C 12 primary alkyl radical; R 6 is a C 1 to C 12 primary alkyl radical or C 2 to C 12 alkylene radical in which the terminal carbons are primary, or at least one of the R 1 -R 2, R 3 -R 4 and R 5 -R 6 combinations forms a heterocyclic radical with the connecting nitrogen atom; X e is an anion selected from the group consisting of fluoride, chloride, bromide, iodide, sulfate, bisulfate, methanesulfonate, carbonate, bicarbonate, phosphate, carboxylate and thiocarboxylate; and n is 1 or 2. (a) an aqueous solution of a salt of thiocarboxylic acid represented by the formula: R-C(=O)-S (-) M (+) wherein R is a branched or straight-chain alkyl containing up to 30 carbon atoms; and M (+) is sodium ion, potassium ion, or ammonium ion with R is a branched or straight-chain alkyl containing up to 30 carbon atoms; and M (+) is sodium ion, potassium ion, or ammonium ion (b) a silane represented by the formula: L f G 2 (-SiZ 3) c wherein G 2 is -CH 2 CH 2 CH 2 -; each occurrence of L is independently chloro group or bromo group; each occurrence of Z is independently RO- or -R, wherein each R is independently defined as above, with the proviso that at least one Z is not -R; c is 1; and f is 1; wherein the salt of the thiocarboxylic acid (a) is provided as an aqueous solution and the reaction is carried out in the presence of a catalytically effective amount of an alkylguanidinium salt phase transfer catalyst having the formula: wherein each of R 1, R 2, R 3, R 4 and R 5 is a C 1 to C 12 primary alkyl radical; R 6 is a C 1 to C 12 primary alkyl radical or C 2 to C 12 alkylene radical in which the terminal carbons are primary, or at least one of the R 1 -R 2, R 3 -R 4 and R 5 -R 6 combinations forms a heterocyclic radical with the connecting nitrogen atom; X e is an anion selected from the group consisting of fluoride, chloride, bromide, iodide, sulfate, bisulfate, methanesulfonate, carbonate, bicarbonate, phosphate, carboxylate and thiocarboxylate; and n is 1 or 2. G 2 is -CH 2 CH 2 CH 2 -; each occurrence of L is independently chloro group or bromo group; each occurrence of Z is independently RO- or -R, wherein each R is independently defined as above, with the proviso that at least one Z is not -R; c is 1; and f is 1; each of R 1, R 2, R 3, R 4 and R 5 is a C 1 to C 12 primary alkyl radical; R 6 is a C 1 to C 12 primary alkyl radical or C 2 to C 12 alkylene radical in which the terminal carbons are primary, or at least one of the R 1 -R 2, R 3 -R 4 and R 5 -R 6 combinations forms a heterocyclic radical with the connecting nitrogen atom; X e is an anion selected from the group consisting of fluoride, chloride, bromide, iodide, sulfate, bisulfate, methanesulfonate, carbonate, bicarbonate, phosphate, carboxylate and thiocarboxylate; and n is 1 or 2.