The intermediate pyrolysis steps and the mechanisms occurring during the conversion of boron-modified polysilazanes (BmPSs) of the type [B(C2H4SiCH3NCH3)3]n (C2H4 = CHCH3, CH2CH2) into amorphous SiBCN ceramics have been investigated. TGA–DTG experiments coupled with mass spectroscopy and gas chromatography have been applied to investigate the mass loss behaviour during ceramization and to receive information on the nature of the escaping gas species. 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. The results contributed (i) to gain structural information on the decomposition process of amorphous SiBCN ceramics during the pyrolytic conversion of processable BmPSs and (ii) to optimize the pyrolytic conversion of BmPSs into SiBCN materials endowed with designed performance properties that surpass those of existing materials. Ceramic conversion starts by the release of methylamine under ammonia atmosphere via transamination and disappearance of BC3 units with the concomitant increased portion of BC3−xNx (0 < x < 3) units in the low temperature regime of the decomposition (RT–200 °C). It is followed by evolution of hydrocarbons due to the cleavage of bonds and formation of radicals that abstract hydrogen. In the high temperature regime of the decomposition, hydrogen is removed due to dehydrocoupling reactions. After heating to 700 °C, BN3, BCN2, and SiN4 domains as well as Si–C–N units of mixed compositions coexist. Further heating to 1000 °C results in a continuous transformation into an amorphous SiBCN network consisting of B(C)N domains, SiCxN4−x units with x = 0, 1, 2 and free carbon.