We developed a new in situ reaction method to synthesize SiC and Si3N4 ceramic solids from meltable precursor compositions into shaped ceramic composites with nanocrystalline grains. The process uses Si powder and 1,2,4,5-tetrakis(phenylethynyl) benzene, which readily react above 1400 degrees C to form the SiC and Si3N4 crystallites in the presence of argon and nitrogen, respectively. X-ray diffraction analysis, Raman spectroscopy and density measurements indicated the formation of near stoichiometric SiC and Si3N4 within the shaped solid. Further characterization of electrical conductivity and oxidative stability of the prepared ceramics analyzed the influence of nanoscale features on intrinsic properties of resulting composites. The hardness and elastic modulus values for the synthesized SiC determined by nanoindentation varied in the range of 25-46 GPa and 300-440 GPa.