The effect of mercapto-modified EPDM (EPDMSH) and thioacetate-modified EPDM (EPDMTA) on the thermal degradation of NR/EPDM (70: 30 wt %) blends has been investigated under anaerobic and aerobic conditions. The anaerobic condition consisted of compression-molding the samples at different times, higher than the optimum curing time established by the oscillatory disk rheometer. The aerobic conditions consisted of ageing the samples in an air circulating oven. EPDMTA in the blend resulted in a reasonable retention of mechanical properties of sample ageing in an air-circulation oven, and a slight increase of crosslink density after ageing under anaerobic conditions. EPDMSH resulted in an accentuated ageing degradation under anaerobic and aerobic conditions. The kinetic parameters of thermal degradation were evaluated from non isothermal TGA experiments taken at different heating rates. The presence of functionalized copolymers in a proportion as low as 2.5 wt % in the blends resulted in a substantial increase of the activation energy, indicating an increase of the resistance against thermal degradation. (c) 2006 Wiley Periodicals, Inc.