An extensive work was performed to analyze how and why electrical properties of carbon black filled rubber compounds change during thermal aging. Several compounds were prepared using different polymer matrices and a broad range of carbon black types. Electrical resistivity of cured and uncured samples was measured, under different conditions, during thermal aging at constant temperature. Other experimental techniques, like transmission electron microscopy, bound rubber measurements and gamma irradiation were also used. Contrary to previous works, it was found that the changes of the electrical resistivity might be explained without any movement of carbon black aggregates during aging. Additionally, a new dependence of the variation of electrical resistivity with carbon black loading is reported and analyzed. This dependence could be the source of the contradictory data and conclusions in previous works. A correlation between the increment of electrical conductivity and the increment of bound rubber during thermal aging was found. This fact suggests that polymer chains and polymer-filler interaction are responsible for the changes during thermal aging instead of aggregate movement. (C) 2003 Elsevier Ltd. All rights reserved.