Silicon-based non-oxide ceramics (SiC, Si3N4) have attracted much attention, primarily due to their good mechanical and chemical properties, and also their reliability at room and elevated temperatures. They have great potential for many industrial uses as engineering components. The addition of a second ceramic (nano)phase to SiC or Si3N4leads to materials which promise applications in many fields and offer solutions for most of the market demands. However, the preparation of these materials is still a challenging task according to the fact that the conventional processes unavoidably lead to size and structure inhomogeneities of the different phases and presence of impurities (because of the use of sintering additives to consolidate the materials) which affect the properties. Here, we propose an alternative strategy using a “ceramic through chemistry” concept. The Polymer-Derived Ceramics (PDCs) route is an attractive means for the design of advanced ceramics; in particular in non-oxide systems. Preceramic polymers are of great interest as they allowed obtaining multi-element ceramics with controlled chemical composition and depending on the composition, they can provide high temperature resistant materials. The principle consists to incorporate one or more elements in the precursor (molecular or polymeric) at molecular scale to ensure compositional homogeneity. For example, boron can be added by specific reactions to polycarbosilazanes to be found after pyrolysis in the final ceramic, i.e., silicoboron carbonitride (Si/B/C/N), in the desired proportion. The type of backbone and functional side chains of precursor molecules influences the ceramic yield, the chemical composition, and the microstructure of Si/B/C/N materials.This is the objective we have fixed in our study. In particular, we demonstrate through FTIR, NMR that we can control the addition of boron in a particular way to deliver after pyrolysis Si/B/C/N materials with tailored structural properties. Furthermore, the boron content plays a key role in the processing of boron-modified polycarbosilazanes. This is demonstrated in the present paper.