The changes in crosslinking. chlorine or sulphur content and unsaturation were determined for three concentrations of hydrogenated acrylonitrile-butadiene rubber (HNBR)/chlorosulfonated polyethylene (CSPE) blends (25:75, 50:50 and 75:25 wt%), and for their individual components. Thermal degradation was carried out over the temperature range from 150 up to 190 degrees C; radiation exposure was performed at three irradiation doses: 100, 200 and 300 kGy. Mechanistic aspects of thermal and radiation ageing are pointed out. It was found that the high activation energy required during the thermal degradation of the studied polymer systems is a consequence of the interaction of mixed components.