The invention relates to a process for the preparation of a rubber composition involving thermomechanically mixing in a preparatory mixing step at least one sulfur vulcanizable elastomer, a particulate filler and an unsymmetrical organosilicon disulfide compound of the formula ##STR1## wherein Z is selected from the group consisting of: ##STR2## wherein R.sup.2 may be the same or different and is independently selected from the group consisting of an alkyl group having 1 to 4 carbons and phenyl; R.sup.3 may be the same of different and is independently selected from the group consisting of alkoxy groups having 1 to 8 carbon atoms and cycloalkoxy groups with 5 to 8 carbon atoms; and R.sup.1 is selected from the group consisting of a substituted or unsubstituted alkylene group having a total of 1 to 18 carbon atoms and a substituted or unsubstituted arylene group having a total of 6 to 12 carbon atoms.