The pyrolysis mechanisms occurring during the conversion of polyborosilazane (PBSZ) into amorphous SiBCN ceramics have been investigated. TGA has been applied to investigate the mass loss behaviour during ceramization. Solid-state ¹¹B, ¹³C and ²⁹Si NMR spectroscopy has been applied to probe the local environment of all NMR active nuclei in the precursor, the thermolysis intermediates and the ceramic residue. IR spectroscopy has been performed to receive valuable information on the chemical bonding in all materials. At temperature below 400℃, NH₃ and H₂ are released via transamination and dehydrocoupling reaction involving N-H and Si-H units. It is followed by evolution of hydrocarbons due to the cleavage of bonds and formation of methane and hydrogen. Ceramization continues at temperature above 800℃. After heating to 1 000℃, ceramization complete and free carbon, BN₃ domains as well as Si-C-N units coexist(SiC_xN_4-x,x=0,1,2,3).