New liquid branched poly(methylvinylborosiloxanes) (br-PMVBS) of random structure were synthesized in three steps. By reacting boric acid with an excess of dimethyldichlorosilane (Me 2 SiCl 2 ) in dry ether a “borosiloxane precursor”: tris(chlorodimethylsilyl) borate B(OSiMe 2 Cl) 3 was prepared. In the second step of synthesis ether solution of B(OSiMe 2 Cl) 3 was added to a mixture of appropriate organic chlorosilanes (Me 2 SiCl 2 , MeViSiCl 2 , MeSiCl 3 , and Me 3 SiCl) and all reagents were reacted with stoichiometric amounts of water, in the presence of pyridine (as an acceptor of HCl), in dry ether, at low temperature (usually at -10 to 0 C). In order to fully react (“to block”) trace silanol groups, reactions of intermediate PMVBS with additional batches of Me 3 SiCl were carried out in the third step, C 5 H 5 N·HCl was filtered off and washed with a dry ether. The solvent was distilled off from filtrates and low molecular weight siloxane oligomers were removed by a vacuum distillation at 130-150 C. Chemical structures of br-PMVBS were confirmed by elemental analysis and spectroscopic methods (FTIR, emission atomic spectroscopy ICP-AES, and NMR: 1 H, 29 Si and 11 B). On the basis of analysis of their 29 Si-NMR spectra the microstructure of polysiloxane chains was proposed. The prepared br-PMVBS had in their structures: triple branching borosiloxane units: BO 1.5 and in some cases methylsiloxane moiety CH 3 SiO 1.5 (T). They contained linkages: Si-O-Si, Si-O-B, vinyl(methyl)siloxane functional groups (CH 2 =CH)MeSiO (Dvi), dimethylsiloxane mers (CH 3 ) 2 SiO (D), and non-reactive trimethylsiloxy terminal groups (CH 3 ) 3 SiO 0.5 (M), but they did not have: hydroxyl functional groups: Si-OH and B-OH, and sensitive to water B-O-B linkages. Molecular weights of br-PMVBS (M n = 1500-3300 g/mol; M w = 3800-7400 g/mol) and their polydispersity (M w /M n = 2.0-2.5) were determined by a size exclusion chromatography (SEC).