The invention relates to compositions based on organosilicon compounds, crosslinkable by condensation reaction, and having controllable modulus, to their preparation and to their use as sealants. The compositions contain as crosslinker(s) at least one organosiloxane containing units of the formula R.sub.dX.sub.eSiO.sub.(4-d-e)/2 (I) where R is preferably a hydrocarbon group and X is a hydrolyzable group, d and e are 0 or small whole numbers, with the proviso that the sum of d+e is .ltoreq.3, from 3 to 15 units of the formula (I) are present per molecule, at least two X radicals are present per molecule and at least one unit of the formula (I) where d=2 and e=0 is present per molecule.