A composite film consisting of poly(3,4-ethylenedioxythiophene) (PEDT) and poly(4-styrenesulphonate) (PSS), obtained by anodic polymerisation on a gold substrate electrode, has been characterised by electrochemical quartz microbalance (EQCM) measurements and impedance spectroscopy (IS), in a non-aqueous electrolyte containing LiClO4 in 1,2-dimethoxyethane (CH3OCH2CH2OCH3). The value of the capacitance was found to be over 25 F cm−3 (5 mF cm−2 for a 2 μm thick film). This confirms the possibility of the use of PEDT/PSS films in electrochemical capacitors. It has been established that lithium ions are sorbed and desorbed reversibly in a potential range corresponding to the oxidised form of the material (+0.4 to −0.2 V vs. Ag|AgCl|0.1 M KClaq). The ionic transport characterised by an effective diffusion coefficient (D eff) was found to be the highest reported for electronically conducting polymers. D eff decreases linearly from 6.5×10−7 to 2.4×10−7 cm2 s−1 in the potential range from +0.4 to −0.2 V vs. Ag|AgCl|0.1 M KClaq while the amount of lithium ions inserted in the film increases. The ionic conductivity equals 1.1×10−5–1.2×10−5 Ω−1 cm−1. The composite films are very stable, being polarised as a cathode in a lithium cell. The open circuit voltage of a cell with a lithium anode and a cathode made of the PEDT/PSS film is 3.37 V.