Positron annihilation lifetime (PAL) spectroscopy has been used to investigate microstructural properties of carbon black–silica–styrene butadiene rubber (SBR) composites. The positron annihilation lifetime shows a direct dependence on the carbon black type as well as the silica content added to the rubber compound. The effect of the tensile strain (ε=0–150%) on the positron annihilation parameters has been investigated by constructing a small load frame to permit the measurement of positron lifetime in situ. The variations in the o-Ps lifetime τ₃ and its intensity I₃ with deformation are discussed in the framework of free volume theories. A simple illustrative sketch for the rubber–filler matrix is proposed.