The present invention relates to a process for preparing multifunctionalized polyorganosiloxanes (POSs) comprising alkoxyl functional siloxyl units and siloxyl units each having an SH functional group, which consists in introducing the SH functional group by the reaction of H.sub.2 S on a precursor POS of formula: A--(RXSiO).sub.a --(RYSiO).sub.b --B where R is especially a C.sub.1 -C.sub.6 alkyl radical, X is a C.sub.1 -C.sub.15 alkoxyl radical, Y is a C.sub.2 -C.sub.30 hydrocarbon group which includes an ethylenically unsaturated group from which the SH functionality derives, a is a number ranging from 1 to 150, b is a number ranging from 1 to 150, the number a+b ranging from 2 to 300 and A and B are terminal groups which are selected respectively, from, in particular, on the one hand (R).sub.3 SiO-- radicals taken alone or as mixtures with (R).sub.2 XSiO-- and/or R(X).sub.2 SiO-- and/or (X).sub.3 SiO-- radicals and on the other hand --Si(R).sub.3 radicals taken alone or as mixtures with --SiX(R).sub.2 and/or --Si(X).sub.2 R and/or --Si(X).sub.3 radicals.