In situ silica reinforcement of styrene–butadiene rubber (SBR) has been achieved by a sol–gel process using tetraethoxysilane (TEOS). SBR was sulfur-cured and the sol–gel reaction of TEOS was carried out in TEOS or in a TEOS–tetrahydrofuran (THF) mixture. The in situ silica was filled homogeneously in the rubber matrix and the size of the in situ silica was influenced by the cross-linking density of the SBR vulcanizate. n-Butylamine was effective for the method using TEOS only, whereas both hydrochloric acid and n-butylamine worked as catalysts under the experimental conditions using THF for the in situ polymerization of TEOS in the SBR vulcanizate. From the viewpoint of the reinforcement of in situ silica and the size stability of the SBR vulcanizate, the method using only TEOS was found to be better than the mixture system. This conclusion was based on the results of tensile tests, dynamic mechanical measurements, optical and transmission electron microscopies.