Acrylonitrile-butadiene copolymers (NBR) with different acrylonitrile contents were melt-mixed with poly(vinyl chloride) (PVC). The morphology of the blends was examined with transmission electron microscopy (TEM). The samples were also exposed to an electron beam and the chlorine-loss upon electron irradiation was monitored with electron dispersion X-ray analyzer (EDX). TEM results indicated that all samples blended with different NBR, including NBR-33 and NBR-41, were heterogeneous, but the dispersion of NBR-33 was the finest. The results of the chlorine-loss study showed that the radiation sensitivity of PVC was improved by the addition of NBR and the extent of improvement was strongly related to the miscibility. Analysis of experimental data demonstrated that chlorine loss of PVC/NBR blends followed the Vesely's double exponential equation but the coefficient and rate constants varied with different NBR. For polymer blends containing a component that-is radiation insensitive and another component that is unstable under irradiation, such as a fluoropolymer and chloropolymer, the loss of fluorine or chlorine as a function of radiation dose can be used to compare the size of the dispersed phase of the polymer blends.