Silica as a reinforcement filler for automotive tires is used to reduce the friction between precured treads and roads. This results in lower fuel consumption and reduced emissions of pollutant gases. In this work, the existing physical interactions between the filler and elastomer were analyzed through the extraction of the sol phase of styrene-butadiene rubber (SBR)-butadiene rubber (BR)/SiO2. composites. The extraction of the sol phase from samples filled with carbon black was also studied. The activation energy (E-a) was calculated from differential thermogravimetry curves obtained during pyrolysis analysis. For the SBR-BR blend, E-a was 3155 kJ/mol. The values obtained for the composites containing 20 and 30 parts of silica per hundred parts of rubber were 231 and 197 kJ/mol, respectively. These results indicated an increasing filler-filler interaction, instead of filler-polymer interactions, with respect to the more charged composite. A microscopic analysis with energy-dispersive spectroscopy showed silica agglomerates and matched the decreasing E-a values for the SBR-BR/30SiO(2) composite well. (c) 2005 Wiley Periodicals, Inc.