Apparent mobilities of positive charge carriers in a regioregular poly(3-octylthiophene) (POT) film at various doping levels are evaluated by an electrochemical technique combining potential-step chronocoulometry and an in situ conductivity measurement. The carrier mobility decreases from 5 × 10−3 cm2 V−1 s−1 to 5 × 10−4 cm2 V−1 s−1 as the doping level increases from 0.014% to 1%. Further electrochemical doping results in a dramatic increase in the mobility, reaching 0.5 cm2 V−1 s−1 at a maximum doping level of 23%. The decrease of the apparent carrier mobilities in the low doping range is ascribed to both scattering of polarons by ionized dopants and the formation of immobile π-dimers which are favored in the highly stacked POT film. The marked increase of mobilities at doping levels beyond 2% is explained by the formation of bipolarons, followed by the evolution of the metal-like conduction.