Exfoliated nanocomposites of poly(3,4-ethylenedioxythiophene) (PEDOT) and montmorillonite(MMT) have been prepared by in situ anodic polymerization, concentrations of clay ranging from 5%w/w to 50% w/w being included in the aqueous polymerization medium. The morphology, electricalconductivity, adherence, thermal stability, charge storage, specific capacitance, electrostability, dopinglevel and band gap have been determined for the different PEDOT–MMT nanocomposites andcompared with those of pristine PEDOT. Many of these properties have been found to depend on boththe concentration of clay and the thickness (micrometric or nanometric) of the generated films. Types Iand II ultracapacitors have been fabricated using nanometric and micrometric films of PEDOT andPEDOT–MMT. The properties of such devices have been characterized and compared with thosereported in the literature for ultracapacitors fabricated using nanocomposites of PEDOT and otherinorganic materials. Both nanometric and micrometric type II ultracapacitors, which correspond to anasymmetric configuration of PEDOT and PEDOT–MMT films, have been found to present the betterproperties (e.g. the specific capacitance for nanometric and micrometric devices is 429 and 116 F g 1,respectively), evidencing the favorable effect of the clay. Finally, the effects of the electrochemicaldegradation on the ultracapacitors have been rationalized using electrochemical impedancespectroscopy.