We propose a new poly-3-methylthiophene (PMeT)-based counter electrode, which is a series of arrays of PMeT-based material dots formed on a SnO2 substrate, and is virtually transparent in a visible region. The electrochromic window (ECW), tin oxide electrode (SnO2 : F)/tungsten oxide film/electrolyte/PMeT-based counter electrode, shows good electrochemical switching behavior, and electrochromic property. The function of the PMeT-based counter electrode is to reserve anions in the electrochromic reaction of the ECW, i.e., a supercapacitor. The behavior of the ECW was studied by a simple equivalent circuit. The EC cell fabricated with the PMeT-based counter electrode (PMeT-EC cell) yielded as large a differential optical density as that fabricated with a carbon-based counter electrode (carbon-EC cell), which we have already proposed, and the PMeT-based counter electrode had a higher capacitance per volume than a carbon-based counter electrode. The initial switching speed of the PMeT-EC cell was, however, slower than that of the carbon-EC cell, especially at lower temperatures. This is due to the fact that the activation energy of the PMeT doping process is higher than that of the carbon charging process. (C) 2003 The Electrochemical Society.