Homogeneous silicon boron oxycarbide (Si-B-O-C) glasses based on SiOxC4-y and BOyC3-y mixed environments were obtained by pyrolysis under inert atmosphere of sol-gel-derived precursors. Their high-temperature structural evolution from 1000degrees to 1500degreesC was followed using XRD, Si-29 and B-11 MAS NMR, and chemical analysis and compared with the behavior of the parent boron-free Si-O-C glasses. The XRD study revealed that, for the Si-O-C and the Si-B-O-C systems, high-temperature annealing led to the crystallization of nanosized beta-SiC into an amorphous SiO2-based matrix. NMR analysis suggested that the beta-SiC crystallization occurred with a consumption of the mixed silicon and boron oxycarbide units. Finally, by comparing the behavior of the Si-O-C and Si-B-O-C glasses, it was shown that the presence of boron increased the crystallization kinetics of beta-SiC.