Organosilicon compounds having at least one .ident.SiH unit per molecule are functionalized via heterogeneous catalysis into, for example, epoxidized POS by hydrosilylation between an ethylenically unsaturated epoxide such as VCMX and a hydrogenated POS of the MDD'M type with M=R.sub.3SiO.sub.1/2, D=R.sub.2--SiO.sub.2/2, D'=RH=SiO.sub.2/2, in which R=alkyl, and wherein the viscosity of the final product is controlled, i.e., to limit the parasitic reactions of cationic polymerization by opening of heterocycles, resulting in partial or total gelling of the reaction mixture, said hydrosilylation characteristically being carried out in the presence of at least one inorganic non-nucleophilic base and, optionally, water.