Measurements of the d.c. electrical conductivity on thermally treated polypyrrole/polyaniline (PPy/PANI) samples, in which the PPy content increased by 10% w.w. starting from pure PANI to pure PPy, followed a sigma(t, T) = sigma(o)(t)exp[-(T-0/T)(1/2)] law. This is consistent with a heterogeneous structure of the granular metal type, in which aging is accompanied by the shrinking of the conductive grains causing the decrease of the sample conductivity, a process which is described by the increase of the parameter T-0. The pre-exponential factor sigma(0)(t) depends on the intrinsic conductivity of the grains and geometrical factors affecting the carrier paths through the energy barriers, as are the grain size distribution and the mean volume occupied by the conducting grains in the material. It was found that for the samples as a whole the thermal aging law, which predicts In sigma(t, T)proportional to t(1/2) is followed for a given temperature T, where t is the time of the thermal treatment, in accordance with a granular metal type structure. On the other hand, the preexponential factor sigma(0)(t) decreases with the aging, following a different law [sigma(t = 0, T)-sigma(t, T)]/sigma(t = 0, T)proportional to t(1/2) where sigma(t = 0, T) is the initial value of sigma(0)(t), that of the fresh sample. This law reveals an aging caused by a degradation proceeding into the interior of the grains in a diffusion-like manner. So, the two different laws of aging, one from T-0 and the other from sigma(0), reveal that the aging does not simply reduce the size of the grains, but affects their interior, this degradation decreases with depth. (C) 2004 Elsevier B.V. All rights reserved.