The effect of Na bentonite, Ca bentonite, and kaolin fillers on the macrostructure and microstructure of acrylonitrile butadiene rubber, ethylene propylene diene rubber, and their blend (50/50) was studied through electrical and mechanical measurements, as well as with positron annihilation lifetime spectroscopy. The real part of permittivity (epsilon'), dielectric loss (epsilon ''), and the crosslinking density were found to increase with increasing filler content. The increase of crosslinking density of the blend with increasing amount of fillers reflects a decrease in the equilibrium swelling up to 21.50 wt % compared with that of the unfilled blends. The mechanical investigation showed pronounced increase in the tensile strength, and in elongation at break with the addition of up to 21.50 wt % of filler. In addition, comparing between different fillers showed that the reinforcing effect of Na bentonite is more effective than Ca bentonite and kaolin but the physico-mechanical of Ca bentonite is less than that for kaolin. The positron annihilation lifetime measurements revealed that the free-volume properties were strongly affected by the amount and type of filler, in particular, the free-volume fraction was dramatically decreased with increasing filler content. Furthermore, correlations were made between the free-volume parameters and both electrical and mechanical properties. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part 13: Polym Phys 47: 1825-1838, 2009