Three novel Si-C-B-N ceramic compositions, namely Si2.9B1.0C14N2.9, Si3.9B1.0C11N3.2 and Si5.3B1.0C19N3.4, were synthesized using the polymer-to-ceramic transformation of the polyorganoborosilazanes [B(C2H4Si(Ph)NH)(3)](n), [B(C2H4Si(CH3)NH)(2)-(C2H4Si(CH3)N(SiH2Ph))](n), and [B-(C2H4Si(CH3)-N(SiH2Ph))(3)](n), where Ph is phenyl (C6H5), at 1050 degreesC in argon. The Si-B-C-N ceramics exhibited significant stability with respect to composition and mass change in the temperature range between 1000 degrees and 2200 degreesC, including isothermal annealing of the samples at the final temperature for 30 min in argon. The mass loss rate at 2200 degreesC was as low as 1.4 wt%.h(-1) for Si5.3B1.0C19N3.4, 1.7 wt%.h(-1) for Si2.9B1.0C14N2.9, and 2.4 wt%.h(-1) for Si3.9B1.0C11N3.2. The measured amount of mass loss rate was comparable to that of pure SiC materials. As crystalline phases, beta -Si3N4 and beta -SiC were found exclusively in the samples annealed at 2200 degreesC at 0.1 MPa in argon. For thermodynamic reasons, beta -Si3N4 should have decomposed into the elements silicon and nitrogen at that particular temperature and gas pressure. However, the presence of beta -Si3N4 in our materials indicated that carbon and boron kinetically stabilized the Si3N4-based composition.