Novel bis-(silylethyl)-oligosulphides are obtained by reacting a vinyl silane of the formula (CH.sub.2 .dbd.CH).sub.m -- SiR.sub.n.sup.1 R.sub.4 -(m+n) in which R represents chlorine, bromine, iodine, C.sub.1 -C.sub.3 -alkoxy, 2-methoxyethoxy, 2-ethoxyethoxy or hydroxy, R.sup.1 represents C.sub.1 -C.sub.3 -alkyl, phenyl or C.sub.5 -C.sub.7 -cycloalkyl, n is 0, 1 or 2, m is 1 or 2 and m+n is 1, 2 or 3 with sulphur as well as in a given case hydrogen sulphide in molar ratios of vinyl silane to sulphur of from 1:2.5 to 1:10 and vinyl silane to hydrogen sulphide of from 1:0 to 1:5 at temperatures between about 100.degree. and 200.degree. C. under pressure of from 1 to 25 bars (total pressure at the reaction temperature) and in the presence of sulphidation catalysts known per se, in a given case followed by hydrolysis or partial hydrolysis of the reaction products. The bis-(silylethyl)-oligosulphides are used as reinforcing additives (adhesive agents) in crosslinkable rubber mixtures containing silicate fillers and in a given case carbon black as well as in a given case sulphur and other customary constituents.