Polymer-laminated electrochromic devices composed of WO3 and Ni(OH)(2) films were fabricated on indium tin oxide (ITO)-coated glass and polyethylene terephthalate (PET) substrates. In each of the devices, data on the transmittance modulance, coloration efficiency, and response time were collected. The transmittance modulation of the glass device was larger than that of the PET device because the large amount of charge was incorporated into each WO3 and Ni(OH)(2) film in glass device. The coloration efficiency, however, was comparable between the two devices, with reversible charge movement and the absence of side reactions. The PET device showed a longer coloring/bleaching performance regardless of whether it had a lower ITO electrical sheet resistance compared to a glass device, which can be attributed to high interfacial charge-transfer resistance. (c) 2005 The Electrochemical Society. All rights reserved.