The thermal and electrochemical properties of polyurethane electrolyte/carbon black composites have been studied. The material structure presented amorphous features and the electrical response was described in terms of equivalent circuits accounting for the different relaxation processes occurring in the medium. The conductivity behavior did not fit satisfactorily either the effective medium theory or conventional percolation models. Flexible electrochemical capacitors using the composite as active material in association with large area collectors have been built and tested. The performance of the capacitors were characterized by a variety of electrochemical techniques, including impedance spectroscopy, cyclic voltammetry, galvanostatic charge–discharge methods, and long term cycling tests. The device prepared with composite electrodes containing 20 wt% of carbon black exhibited a capacity of 0.7 F cm−3 under 2 V providing an energy density of 0.4 W h l−1.