The design, synthesis, and properties of two new types of poly(borosiloxane) preceramic polymers, diethylborazine- (PVS−DEB) and pinacolborane-modified (PVS−PIN) poly(vinylsiloxane)s, are reported. The polymers are synthesized in excellent yields by the RhH(CO)(PPh3)3-catalyzed reactions of poly(vinylsiloxane) (PVS) with either diethylborazine (DEB−H) or pinacolborane (PIN−H). The spectroscopic data for the polymers, along with results of the RhH(CO)(PPh3)3-catalyzed hydroboration reactions of diethylborazine and pinacolborane with the model compound vinyltris(trimethylsiloxy)silane, are consistent with predominately anti-Markovnikov hydroboration of the polymer vinyl groups by the boranes. The degree of polymer hydroboration is simply controlled by varying the reactant ratios with the compositions of the PVS−DEB polymer ranging from (MeSiO1.5)0.42(PhSiO1.5)0.37(ViMe2SiO0.5)0.17(DEB(CH2)2Me2SiO0 .5)0.04 to (MeSiO1.5)0.42(PhSiO1.5)0.37(DEB(CH2)2Me2SiO0.5)0.21 and those of the PVS−PIN polymers from (MeSiO1.5)0.42(PhSiO1.5)0.37(ViMe2SiO0.5)0.18(PIN(CH2)2Me2SiO0. 5)0.03 to (MeSiO1.5)0.42(PhSiO1.5)0.37(PIN(CH2)2Me2SiO0.5)0.21. The borane-modified polymers are soluble and stable in ethers, acetone, methylene chloride, benzene, and pentane. Molecular weight analyses using SEC with refractive index, viscometry, and light-scattering triple detection show significant differences in hydrodynamic volume, molecular weight, radius of gyration, and conformation, which indicate that the PVS−PIN and PVS−DEB polymers adopt a more branched and spherical structure compared to the unmodified PVS. Bulk pyrolysis of the modified resins produced SiOCB ceramic chars with boron contents ranging from 0.3 to 0.7% depending upon the percentage of polymer modification. XRD and TEM studies of the 1800 °C chars of PVS and PVS−PIN showed significant differences in the degree of grain growth and distribution with the boron-modified char showing both a smaller average crystallite size and a more narrow size range. The PVS−PIN polymers were found to be excellent melt-processable, single-source precursors to small-diameter SiOCB fibers.