The performance of polyaniline-based electrochemical capacitors was evaluated under various experimental conditions. The capacitor consisted of two platinized tantalum foils coated with polyaniline as the active material, a separator, and an appropriate aqueous electrolyte solution. The polyaniline coatings were formed galvanostatically 5 mA cm(-2) from a 0.1 M aniline +1.0 M HCl aqueous solution. With a polyaniline lending formed by a deposition charge of 20 C cm(-2) on each electrode and with a 4.0 M HBF4 aqueous solution as the electrolyte for an optimized capacitor, energy and power densities of 2.7 Wh kg(-1) and 1.0 kW kg(-1) (of active polymer) were achieved, respectively. Cyclic voltammograms for both positive and negative polyaniline electrodes of the capacitor before and after 20,000 cycles showed only a 5% loss of polyaniline electroactivity, which was smaller than the observed 33% decrease in the discharge capacity of the capacitor. Cyclic voltammetry and electrochemical impedance spectroscopy were used to characterize the capacitors and to understand their initial performance loss upon constant-current cycling.