The synthesis of purified functionalized organosilicon compounds containing at least one active azo group having formula (I): [(G0)3SiO1/2]m[(G0)2SiO2/2]n[G0SiO3/2]o[SiO4/2]p[(G2)a(G1)a′(Z—CO—HN═NH—CO-A)SiO(3-a-a′)/2] entails providing at least one hydrazino (—HN—NH—) precursor (II) of the compound (I) and oxidizing the precursor (II) into an azo group specific to the compound (I) with the aid of an oxidizing system comprising at least one oxidant (Ox) and at least one base (B), wherein 1) Ox is selected from among the oxidants that can oxidize a hydrazine function into an azo function, with or without the exclusion of N-bromosuccinimide (NBS), 2) and/or Ox (optionally B) is/are employed in stoichiometric quantities in relation to the precursor (II); 3) and/or the organosilicon compounds (I) thus prepared are subjected to a purification post-treatment to eliminate any unwanted residues of the base B used, with the proviso that, when NBS is not excluded, characteristic (1) is combined with characteristic (2) and/or characteristic (3).