The CPE/NR blends with blend ratio of 50/50% by weight were prepared with various blending conditions. The resistance to oil and thermal aging of the blends was investigated and correlated with phase morphology. The NR dispersed phase size in blends was found to decrease with increasing rotor speed up to 45 rpm and subsequently level off at higher rotor speeds. With increasing mixing time at a given rotor speed, NR dispersed phase size reached a minimum and increased again with a further increase in mixing time, caused by domain breakup and phase coalescence, respectively. In addition, the results revealed a strong relationship between NR phase size and resistances to oil as well as thermal aging, that is, the smaller the dispersed phase size, the higher the oil resistance and the thermal aging properties. (C) 2003 Wiley Periodicals, Inc.