ESR studies have been carried out on ClO4--doped polyfuran (PF) and BF4--doped polythiophene (PT) films synthesized by constant potential electrolysis. Variation of spin concentration and linewidth of ESR signal with temperature were investigated. The temperature dependence of conductivity supported the Mott's variable range hopping mechanism for PT and PF freestanding films. Temperature dependence of activation energy indicated that hopping is the dominating mechanism of charge transport. (C) 2000 Elsevier Science Ltd. All rights reserved.