The phase morphology and oil resistance of 20/80 NR/NBR blends filled with different types of fillers and copolymers were investigated. In the case of filler effect, N220, N330, and N660 carbon blacks with different particle sizes were used. Additionally, the blends filled with non-black-reinforcing fillers, that is, precipitated and silane-treated silica, were investigated. To study the compatibilization effect, maleated ethylene propylene diene rubber (EPDM-g-MA) and maleated ethylene octene copolymer (EOR-g-MA) were added to the blends. The results revealed that the addition of filler, either carbon black or silica, to the blend caused a drastic decrease in NR dispersed phase size. Carbon blacks with different particle sizes did not produce any significant difference in NR dispersed phase size under the optical microscope. Silica-filled blends showed lower resistance to oil than did the carbon black-filled blends. In addition, it was determined that neither EOR-g-MA nor EPDM-g-MA could act as a compatibilizer for the blend system studied. The oil resistance of the blends with EPDM-g-MA is strongly affected by the overall polarity of the blend. In the case of EOR-g-NIA, the oil resistance of the blends is significantly governed by both overall polarity of the blend and phase morphology. (C) 2003 Wiley Periodicals, Inc.