Amorphous hydrous ruthenium oxide thin films have been investigated and demonstrated to be an excellent optical modulating material as well as an electrochemical supercapacitor. The a-RuO2.OxH(2)O films have been prepared by cyclic voltammetry on ITO-coated glass substrates from an aqueous ruthenium chloride solution. The cyclic voltammograms of this material show proof of capacitive behavior including two redox reaction peaks in each cathodic and anodic scan. The as-deposited a-RuO2.xH(2)O films exhibit an optical density change of 0.3 at 670 nm wavelength with capacitor charge/discharge in an anodic coloring fashion. This paper describes our electrochemical and electro-optical characterization results and analyses. (C) 2002 The Electrochemical Society.