The stability of conductivity in FeCl3-doped polymer blends of poly(3-octylthiophene) and ethylenevinylacetate has been studied over a wide temperature range (10–410 K). In contrast to the large conformational changes in undoped materials the dc conductivity behaviour was found to be practically reversible and stable up to the melting point of ethylenevinylacetate (350 K). A spontaneous conductivity increase above 350 K was interpreted to be related to additional doping by dopant molecules trapped inside a network of partially crosslinked poly(3-octylthiophene). Crosslinking of matrix polymer by electron irradiation did not improve the stability of conductivity. The electrical properties of heavily irradiated samples indicated an increase in the average size of conducting islands which was related to an irradiation-induced crosslinking of poly(3-octylthiophene) aggregates.