Oxymethylene-linked polyoxyethylene complexed with LiClO4 at a Li/EO molar ratio of 0.05 has a conductivity of 4.7 x 10(-4) S.cm(-1) at 25 degrees C and a transmittancy of 90%. It is a promising electrolyte for an all-solid-state complementary electrochromic window using WO3 and NiOx. The modulation depth of the window exceeds 50% from 700 nm to 900 nm. The electrochromic ability of the window increases with temperature and lithium salt concentration and the window also exhibits good storage property under open circuit. Current and optical responses of the window as well as the electrochromic process are also discussed.