A new approach to the synthesis of highly branched alkoxy functionalized polysiloxanes has been developed. Dehydrocarbon polycondensation of tetraalkoxysilanes with 1,1,3,3-tetramethyldisiloxane ((MMH)-M-H) catalyzed by tris(pentafluorophenyl)borane [B(C6F5)(3)] leads to highly branched organopolysiloxanes substituted with alkoxy groups at silicon atoms. These copolymers could be used as silanol-free hydrophobic silicone resins. Polyconderisations of (MMH)-M-H with tetramethoxysilane (TMOS) and with tetraethoxysilane (TEOS), catalyzed by B(C6F5)3, were studied. Use of TMOS or TEOS lead to stable, Soluble. branched copolymers containing, a great number of reactive alkoxy groups. Dehydrocarbon polycondensation is accompanied by a metathetic exchange Of functional groups as well its cleavage of siloxane bonds adjacent to the Si-H group. Both of these reactions, as well as dehydrocarbon polycondensation, occur via the tertiary silyloxonium borate intermediate.