The dc electrical conductivity (σ) of HCI-protonated polyaniline, polypyrrole, and their blends was measured from 80 to 300 K for thermal aging times between approximately 0 and 600 h. The thermal aging took place at 70° C under room atmosphere. The change of σ with the temperature (T) and the decrease of σ with the thermal aging time (t) are consistent with a granular metal type structure, in which conductive grains are randomly distributed into an insulating matrix. Aging makes the grains strink in a corrosion-like process. From σ = σ(T) measurements the ratio s/d, where s is the average separation between the grains and d their diameter, as well as the rate d(s/d)/dt of their decrease with t were calculated. These revealed that the conductive grains consist of a shell, in which aging proceeds at a decreasing rate, and a central core, which is consumed at a much slower rate. Our measurements not only permitted the estimation of the shell thickness, which lies between 0 and 5 &ANGS;, but also gave quantitative information about the quality of the shells and the cores from their aging rates. The shells are consumed with an average rate of d(s/d)/dt = 6.6 X 10(-4) (h(-1)), which is about 5 times greater than the more durable cores. © 2005 Wiley Periodicals, Inc.