Resorcinol–formaldehyde/silica composite (RF/SiO₂) gels were acid-catalyzed formed in one pot at 27°C within 60min, and then dried to aerogels with supercritical fluid CO₂. After carbonization in nitrogen atmosphere and a magnesiothermic reaction at 700°C, RF/SiO₂ aerogels were successfully converted to monolithic mesoporous silicon carbide (SiC aerogels). The starting RF/SiO₂ aerogels had an interpenetrating organic/inorganic network and the resulting SiC products preserved monolithic mesoporous morphology similar to the original templates. The as-synthesized SiC aerogels consisted of nanocrystalline β-SiC, possessed a BET surface area of 232m²/g and showed sufficient mesoporosity. They had a direct band gap of about 3.2eV (less than that of bulk β-SiC) and showed photoluminescence at room temperature. The mechanisms about the formation of RF/SiO₂ gels and the conversion to SiC aerogels were discussed. Potentially, the reported method can be used to convert many metal or semimetal oxide/carbon composite aerogels to carbide aerogels at relatively low temperature for many catalytic, electronic, photonic and thermal applications.