Silicon-based non-oxide ceramics (SiC, Si3N4) have attracted much attention due to their good properties and reliability at high temperatures which can be enhanced by addition of a second ceramic (nano)phase. However, their preparation is a challenging task as conventional processes lead to inhomogeneities and impurities affecting the properties. The Polymer-Derived Ceramics (PDCs) route is an alternative strategy using a “ceramic through chemistry” concept. Preceramic polymers have allowed obtaining multi-element ceramics with controlled chemical composition by incorporating elements to provide high temperature resistant materials. In this study, boron is added to polycarbosilazanes to obtain silicoboron carbonitride (Si/B/(C)/N) ceramics after pyrolysis. We investigate through FTIR, solid-state NMR, the chemistry of boron-modified polycarbosilazanes as well as their pyrolysis behavior combining TG experiments and solid-state NMR. By controlling the boron content in the polymer at molecular scale, we can deliver after pyrolysis bulk Si/B/(C)/N materials with tailored properties. The high temperature behavior is investigated by thermogravimetric analysis, XRD, elementary analysis and Raman spectroscopy and we show how the boron content and the nature of the atmosphere affect the structural evolution of the Si/B/(C)/N phase at high temperature.