We have investigated the optical properties of an all-thin-film electrochromic device, with a thin film of ZrO2 acting as an ion conductor. The device also employed electrochromic layers of amorphous or crystalline WO3 and NiVxOyHz. Transmission (T) and reflection (R) spectra were recorded in the wavelength range 300-2500 nm at different intercalation levels, both for single films and complete devices. The results show that T decreases significantly upon intercalation in the WO3 thin films as well as in the devices. The reflectance only shows minor changes. (C) 2004 Elsevier B.V. All rights reserved.