Apparent mobilities of positive charge carriers in a regioregular poly(3-octylthiophene) film in varying oxidation states were successfully evaluated by using electrochemical techniques. Electrochemical ClO4 - doping caused a decrease of the mobility by about one order of magnitude until the doping level reached about 2%. Further electrochemical doping resulted in the drastic increase of the mobilities up to 0.5 cm2 V-1 s-1 at a maximum doping level of 23%.