In situ silica filling of butadiene rubber (BR) was carried out by the sol-gel process using tetraethoxysilane (TEOS). BR was sulfur-cured, and the resultant crosslinked BR was subjected to soaking in TEOS at 30 degrees C for 48 h. The swollen vulcanizates were kept in the aqueous solution of n-butylamine at 30 degrees C for 24 h, followed by standing at 50 degrees C for 72 h to conduct the in situ sol-gel reaction of TEOS. The resultant vulcanizates contained silica particles which were prepared by the in situ filling technique. They were subjected to tensile and dynamic mechanical measurements, differential scanning calorimetry and transmission electron microscopic observation. The in situ formed silica was dispersed homogeneously in the rubbery matrix. The size of in situ silica was influenced by the crosslinking density, probably because it was formed in the crosslinked rubber networks. Compared to the conventional silica-filled BR vulcanizate, which was prepared by mechanical mixing of the silica particles, the new vulcanizates with the in situ formed silica showed better mechanical properties.