Poly(vinyl chloride)-nitrile rubber blends undergo a self-crosslinking reaction (crosslinking without the aid of any external curing agents) during processing at elevated temperatures, resulting in an increase in torque, as studied by means of a Brabender Plasticorder. The extent of the reaction depends on the blend composition, processing temperature, shear rates, fill factor and the presence of a PVC stabilizer. The chemical interaction is analysed by infrared spectroscopy, which shows that the reaction involves the hydrolysis of the nitrile group by HCl liberated during the degradation of PVC.