The introduction of B atoms in SiOC glass networks has been achieved through the pyrolysis of sol-gel-derived polyborosiloxanes under an inert atmosphere. The starting gels were obtained from hydrolysis-condensation reactions of triethyl-borate (B(OEt)(3)) and an organically modified trialkoxysilane (EtSi(OEt)(3)). The resulting hybrid EtSiO1.5-B2O3 gels showed a homogeneous dispersion of the B atoms in the siloxane network via equivalent to Si-O-B= bonds. The presence of such borosiloxane bridges prevents the formation of cyclic or cage siloxane entities and leads to relatively high ceramic yields (similar to 80%). The transformation of the polyborosiloxanes into amorphous SiBOC glasses was followed using Fourier transform infrared spectroscopy and multinuclear magic-angle spinning-nuclear magnetic resonance (MAS-NMR) (B-11, C-13, and Si-29). An important change in the carbon, silicon, and boron environments occurs during pyrolysis. Interestingly, the B-11 MAS-NMR spectra suggest a progressive replacement of the B-O bonds by B-C bonds, which leads to a distribution of trigonal BCxO3-x sites in the glass that was pyrolyzed at 1000 degreesC, with a residual amount of B(OSi)(3) sites. The resulting glasses can thus be described as silicon-boron oxycarbide networks that are based on SiCxO4-x and BCyO3-y mixed environments.