The electrical properties of poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) films were improved by ethylene glycol (EG). Solid tantalum electrolytic capacitors (STEC) fabricated using such EG-treated PEDOT:PSS films as the cathode electrodes exhibited high performances. The equivalent series resistance (ESR) and capacitance-frequency performance of the STEC were significantly improved comparing to those without EG treatment. Particularly, the leakage current (LC) was lowered at the same time.