We proposed a carbon-based counterelectrode for electrochromic windows (ECWs) and fabricated a new solid-state ECW consisting of an indium tin oxide electrode (ITO, IN2O3:SN)/a WO3 film/a polymeric solid electrolyte (PSE)la carbon-based counterelectrode. The carbon-based counterelectrode is a series of arrays of carbon material dots formed on an ITO substrate and is virtually transparent in a visible region, Those carbon dots play a part in the formation of an electric double layer in an electrochromic reaction of the ECW. The electric double layer capacitance of the counterelectrode increases linearly as a function of carbon-dot covering percentage on the ITO substrate. Maximum differential optical density of the ECW increases with the covering percentage of the carbon dots up to a point and levels off for further increase in the covering percentage. The response time of coloration decreases with temperatures, which is caused chiefly by the temperature dependence of an ionic conductivity of the PSE. The behavior of ECWs is explained well with a simple equivalent-circuit comprising two capacitors corresponding to the WO3, film and the carbon-based counterelectrode, an electric resistor of the PSE and a power source connected in series.