The present communication is concerned with the effect of the carbon source on the microstructure and mechanical properties of reaction bonded boron carbide composites. The composites were fabricated by molten Si-infiltration of partially sintered boron carbide preforms with 20, 30, or 40 vol.% porosity with and without free carbon addition. The infiltrated composites consist of four phases namely the original boron carbide particles, the ternary B 12(B,C,Si)3 compound, formed in the course of the infiltration process, β-SiC and residual silicon. In the absence of initial free carbon, the β-SiC phase appears as plate-like particles. In the presence of initial free carbon, the β-SiC phase particles display an irregular polygonal form. The plate like morphology of the SiC phase improves significantly the strength, the fracture toughness and reliability of the infiltrated composites. For an equal volume of SiC, the high aspect ratio of the plate-like particles increases their number per unit volume and thereby, the number of boundaries that a propagating crack has to cross. Moreover, crack deflection on SiC plates was also observed.