In situ composite particles of titanium diboride/boron carbide (TiB2/B4C) and zirconium diboride/boron carbide (ZrB2/B4C) were synthesized from low-cost boric acid and sucrose-based precursor solution with the addition of titanium (IV) ethoxide and zirconium oxynitrate, respectively. The reaction was conducted at 1650 degrees C for 90 min under Ar (240 L/h) + H-2 (10 L/h) gas flow. The characterization of the resultant powders was performed in terms of phases in particles and their microstructures and elemental mapping. The particles synthesized from Ti-based precursor comprised hexagonal TiB2 platelets and polyhedral B4C particles while those synthesized from Zr-based precursor comprised equiaxed polyhedral ZrB2 particles with slightly elongated polyhedral B4C particles. As Ti and Zr concentrations increased in the precursor solution, the hexagonal lattice parameters of B4C and its corresponding lattice volume decreased. Additionally, the results provide evidence that the solid solubility of C in TiB2 and ZrB2 phases is relatively low.