Bulk SiBCN ceramics derived from polyborosilazanes of the type [B(C2H4SiRNH)3]n (1a, R = CH3; 2a, R = H; C2H4 = CHCH3, CH2CH2) exhibit an exceptional structural stability at high temperature. Therefore, such quaternary systems are of great scientific and technical interest as fibrous reinforcements intended for high-temperature applications. In this context, the design of novel polyborosilazanes, which display properties tailored for the preparation of SiBCN fibers, is studied. Boron-modified polysilazanes of the type [B(C2H4SiRNCH3)3]n (1b, R = CH3; 2b, R = H) are prepared via aminolysis of the tris(dichlorosilylethyl)boranes B(C2H4SiRCl2)3 (1, R = CH3; 2, R = H). It is shown that the functionalisation of the precursors with NCH3 units improves their processability (i.e. solubility) compared to that of their ammonolysed analogs [B(C2H4SiRNH)3]n (1a, R = CH3; 2a, R = H). In addition to the influence of the NCH3 units, the presence of the SiCH3 functions in such polymers offers the best potential for the preparation of fibers by melt-spinning. As-spun fibers are then converted under controlled atmosphere into high-temperature stable SiBCN fibers according to the polymer-derived ceramic route.