The results of moving die rheometry studies show that in contrast to sulfur vulcanization of carboxylated nitrile rubber (XNBR), where the loss component of the torque (S") decreases with curing time, the zinc oxide (ZnO) crosslinking of XNBR is accompanied by a gradual increase in S", which becomes more pronounced in the presence of reinforcing high abrasion furnace carbon black filler. In dynamic mechanical studies, the sulfur-vulcanized XNBR shows a single peak around -5-degrees-C. The peak height reduced in the presence of the reinforcing filler with no shift in the peak position. However, the ZnO-vulcanized XNBR shows one extra peak around 54-degrees-C, in addition to the main transition around -5-degrees-C. Incorporation of reinforcing filler causes lowering of the peak height of the main transition, while the peak height of the high-temperature transition gradually increases. For the crosslinking system consisting of both sulfur and ZnO, the characteristics of the high-temperature transition are retained. These results have been explained on the basis of the formation of ionomers of XNBR during crosslinking by ZnO. Reinforcing carbon black filler favours the stabilization of these ionomers.