The paper reports on detailed investigations of the synthesis, phase evolution and high temperature properties of ceramics derived from boron-modified polysilylcarbodiimides. The polymeric precursors were obtained by a dehydrocoupling reaction of tris(hydridosilylethylborane), B(C2H4-SiH3)(3) (C2H4 = CHCH3, CH2CH2), with different amounts of cyanamide, H(2)NCdropN. Thermolysis of the precursors, which was monitored by thermogravimetric analysis (TGA) delivers Si-B-C-N ceramics with chemical compositions directly influenced by the molecular structure and composition of the precursors. High temperature investigations using TGA of as-received ceramics and XRD of annealed samples point out that thermal stability is predominantly a function of the H(2)NCadropN : B(C2H4-SiH3)(3) ratio used in precursor synthesis. Less stable ceramics decompose around 1670degreesC, while thermally stable materials withstand degradation at least up to 1950 2000degreesC. TEM investigation indicates that upon heat treatment SiC/Si3N4 nano/nano composites are generated at approximately 1800degreesC and resist microstructural changes up to at least 1900degreesC.