The dynamic mechanical analyses of moulded samples of carboxylated nitrile rubber (XNBR) in the presence of zinc oxide show occurrence of a biphasic structure in both unfilled and filled compositions; two transitions appear in elastic modulus (E＇) and loss tan delta with temperature, The low temperature transition is believed to be due to the T-g of the XNBR matrix and the high temperature transition is believed to be due to the hard phase of the ionic aggregates of zinc carboxylic acid salt. Incorporation of a filler like calcium carbonate and clay affects the dynamic mechanical properties, particularly the high temperature transition; the effect is more pronounced in the clay filled composition.