Conducting composites were prepared by melt mixing of ethylene-propylene-diene terpolymer (EPDM) or styrene-butadiene rubber (SBR) and 35 wt % of carbon black (CB). Stability of electrical properties of rubber/CB composites during cyclic thermal treatment was examined and electrical conductivity was measured in situ. Significant increase of the conductivity was observed already after the first heating-cooling cycle to 85 degrees C for both composites. The increase of conductivity of EPDM/35% CB and SBR/35% CB composites continued when cyclic heating-cooling was extended to 105 degrees C and 125 degrees C. This effect can be explained by reorganization of conducting paths during the thermal treatment to the more conducting network. EPDM/35% CB and SBR/35% CB composites exhibited very good stability of electrical conductivity during storage at ambient conditions. The electrical conductivity of fresh prepared EPDM/35% CB composite was 1.7 x 10(-2) S cm(-1), and slightly lower conductivity value 1.1 X 10(-2) S cm(-1) was measured for SBR/35% CB. The values did not significantly change after three years storage. (C) 2009 Wiley Periodicals, Inc. J AppI Polym Sci 112: 2918-2924, 2009