Novel densely crosslinked polycarbosiloxanes were obtained by using functional branched prepolymers. Two types of soluble prepolymers were prepared from di- and trifunctional alkoxysilane monomers via cohydrolysis/condensation and for both final crosslinking occurred via hydrosilylation. The prepolymers having only vinyl functionalities (poly[phenylmethylvinyl]siloxanes, system A) were crosslinked by using a crosslinking agent with reactive silicon-hydrogen bonds. In the prepolymers having both silicon-vinyl and silicon-hydrogen functionalities (poly[phenylmethyl-vinylhydro]siloxanes, system B) crosslinking took place intermolecularly. For the characterization of the prepolymers H-1-NMR, Si-29-NMR, FT-IR spectroscopy, analytical SEC and VPO were employed. The prepolymers were fractionated with preparative SEC and the fractions analyzed with H-1-NMR and analytical SEC. The crosslinking reaction was followed by FT-IR spectroscopy. The polymer networks were fully transparent homogeneous materials and are promising for future optical applications.