Neutral poly(3,4-ethylenedioxythiophene) (PEDOT) thin films can be switched to an electronically conducting form either by oxidation (p-doping) or reduction (n-doping) in anhydrous organic solvents. The maximum attainable n-conductivity is ca. 1% of the maximum p-conductivity. However, based on spectroelectrochemical and in-situ conductance measurements, the p-conductivity regime can be divided into two domains, in which either positive polarons or bipolarons and free carriers are the major charge carriers. In the n-conductivity regime, voltammetric, spectral, and conductance data suggest only the generation of negative polaron-type carriers. These results imply that the conductivity due to positive or negative polarons is of the same order of magnitude and that the higher maximum p-conductivity may be attributed to the generation of other charge carriers in the highly stable oxidized PEDOT films. The reduced form is not stable even in an extremely dry oxygen-free environment, which severely hampers the use of n-doped PEDOT in practical applications. The conductivity of pristine PEDOT films can be markedly enhanced by successive p-doping and undoping. In addition, the conductance slowly increased after an anodic potential step. The origin of these effects is not known.