The doping/dedoping behavior of poly(3,4-ethylenedioxythiophene) (PEDT) in acetontrile (lithium, sodium, and tetrabutyl perchlorate) and aqueous electrolytes (lithium and sodium perchlorate, sodium nitrate, sulfate, citrate, and toluenesulfonate) was investigated using the electrochemical quartz crystal microbalance (EQCM) technique. No significant cation exchange was observed. For the investigated anions (except sulfate), the calculated exchanged molar mass could be explained by the reversible replacement of solvent by anions during oxidation. The special behavior of sulfate that carries water into the film was explained by the high charge-to-volume ratio of this anion.