A new composite film was prepared by depositing hydrous ruthenium oxide (RuO2) particles into a polymer matrix comprising of poly(3,4ethylenedioxythiophene) (PEDOT) and poly(styrene sulfonic acid) (PSS). The composite, PEDOT-PSS-RuO2, showed an enhanced capacitance and optical properties due to the presence of RuO2 particles in the composite. Specific capacitance of the PEDOT-PSS-RuO2 composite was found to be 1409 F/g. After anodic polarization, the PEDOT-PSS-RuO2 composite film exhibited enhanced visible-light coloration in comparison with PEDOT/PSS and simple RuO2 films. Differential voltabsorptometric curves (dA/dt versus potential (derivative cyclic voltabsorptogram, DCVA)) were deduced for few selected wavelengths to characterize the optical properties. The changes in the electrochromic characteristics (in the visible region) of the composite is attributed mainly to transition of Ru(III) to Ru(IV). SEM images revealed the presence of smaller sized particles of hydrous ruthenium oxide in the composite film. The existence of smaller size RuO2 particles having more active surface area is the reason for the enhanced capacitance and optical properties. (c) 2005 Elsevier Ltd. All rights reserved.