Samples of a B<sub>4</sub>C/SiC composite were fabricated by infiltrating liquid Si into a preform of B<sub>4</sub>C, SiC, residual C, and about 20-25 vol% of pores. A detailed microstructural investigation revealed a dense composite product consisting of phases of SiC, B<sub>4</sub>C, residual Si, plate-like β-SiC within B<sub>4</sub>C, clusters of preexisting SiC grains shrouded by reaction formed SiC, and B<sub>12</sub>(Si,B,C)<sub>3</sub> surrounding some B<sub>4</sub>C. A thermodynamic assessment suggested a plausible mechanism for the phase formation and microstructural evolution. Limited C dissolved in Si at high temperature led to the growth of existing SiC while in parallel, the molten Si also reacted with C to form additional SiC grains. At high temperature, B<sub>4</sub>C reacts with liquid Si to form β-SiC while at cooling, B<sub>12</sub>(Si,B,C)<sub>3</sub> formed to cover the original B<sub>4</sub>C as a rim in addition to continued formation of β-SiC crystals. Unreacted molten Si would eventually solidify to exist as residual Si.