The effects of metal oxide (PbO) on covulcanization of the blend [carboxylated nitrile rubber (XNBR)-Polyacrylic rubber (ACM) = 50 : 50] in the presence of 2-mercaptobenzothiazole (MBT) or ethylene thiourea (ETU) have been evaluated to elucidate the curing mechanism. The optimum mole ratio of ETU-PbO and MBT-PbO with respect to cure characteristics and physical properties have been found to be 1 : 1.5 and 1 : 0.5, respectively. The examination of infrared (IR) spectra reveals that crosslinking occurs at the C-Cl cure site of the ACM and COOH cure site of the XNBR. Based on the optimum mole ratios and spectral analysis a scheme of reactions has been proposed.