A temperature variation of de conductivity in the range 77-300 K has been carried out in order to explore the mechanism of charge transport in polyaniline (PAN) doped with sulfuric acid. The variable range hopping (VRH) exponent changes a's the transition of the PAN lattice takes place in a narrow pH range thereby indicating that the charge transport is crucially composition dependent. A decrease in activation energy has been observed as the doping level is increased. Spin concentration of charge carriers determined by electron spin resonance spectroscopy has also been found to depend on the doping level of the specimen. Polarons and bipolarons formed during the doping process are the charge carriers in this system. The temperature dependence of de conductivity and activation energy data are indicative of existence of both VRH and mixed conduction for various doping levels in these samples. (C) 2002 Elsevier Science B.V. All rights reserved.