The electrochemistry of poly(3,4-ethylenedioxythiophene) (PEDOT) was studied in two ionic liquids with bulky organic anions, i.e., 1-butyl-3-methylimidazolium (BMIM) diethylene glycol monomethyl ether sulfate (MDEGSO(4)) and BMIM octyl sulfate (OctSO(4)). BMIM-MDEGSO(4) is a liquid, while BMIM-OctSO(4) is in solid form at room temperature. Electrosynthesis of PEDOT in BMIM-MDEGSO(4) with an MOT concentration of 0.1 M and in BMIM-MDEGSO(4)/EDOT 1/1 (w/w) solution resulted in no polymer at all or a very limited amount of polymer on the electrode surface, as determined by cyclic voltammetry in 0.1 M KCl(aq) solution. In contrast, electrosynthesis of PEDOT in BMIM-OctSO(4)/EDOT 1/1 (w/w) resulted in a high yield of electroactive material on the electrode surface. Furthermore, electrosynthesis of PEDOT in ionic liquid-water solution (C-ionic liquid = 1.5 M) containing 0.1 M MOT was also found to give a relatively high yield of electroactive material on the electrode surface, both for 1.5 M BMIM-MDEGSO(4)(aq) and 1.5 M BMIM-OctSO(4)(aq). The PEDOT electrodes showed an anionic potentiometric response in 10(-5)-10(-1) M KCl(aq) solution, indicating a predominant anion transfer at the polymer-solution interface despite the relatively bulky anions (MDEGSO(4)(-) or OctSO(4)(-)) incorporated as counterions in PEDOT during electropolymerization. On the basis of electrochemical impedance spectroscopy, the charge (ion) transport properties of the polymer film were strongly influenced by the water content of the ionic liquid (C-ionic liquid 0.05-2.0 M).