The free-volume properties and miscibility of ethylene-propylene-diene monomer/acrylonitrile-butadiene rubber blends with poly(vinyl chloride) used for compatibilization were investigated with positron annihilation lifetime spectroscopy and Doppler broadening of annihilation radiation. The results showed that the ortho-positronium annihilation lifetimes and intensities as well as the S parameter had a linear relationship with a negative slope as a function of the weight percentage of acrylonitrile-butadiene rubber, which indicated the miscibility of the blend. The filling effect of silica on the free-volume properties of an ethylene-propylene-diene monomer/acrylonitrile-butadiene rubber (75/25) blend was also examined. On the other hand, a correlation between the size and concentration of the free-volume holes and the electrical and mechanical properties of the aforementioned blends was established. (c) 2007 Wiley Periodicals, Inc.