Mercaptoalkylalkoxysilane is obtained by reacting at least one sulfide of the general formula MHS wherein M is an alkali metal or ammonium with a haloalkyl silane in an aqueous reaction medium in the presence of an acidic gas to maintain the pH of the reaction medium at or below about 10, and in the additional presence of alkylguanidinium salt phase transfer catalyst to provide mercaptoalkylalkoxysilane, the alkylguanidinium salt phase transfer catalyst being represented by the general formula: ##STR00001## wherein each of R.sup.1-5 is a primary alkyl radical and R.sup.6 is a primary alkyl or bis(primary alkylene) radical, or at least one of the R.sup.1 R.sup.2, R.sup.3 R.sup.4 and R.sup.5 R.sup.6 combinations with the respective connecting nitrogen atom forms a heterocyclic radical; X is an anion; and n is 1 or 2.