Electrochromic switchable mirrors consisting of a multilayer of Mg(4)Ni/Pd/Al/Ta(2)O(5)/H(x)WO(3)/indium tin oxide on a glass substrate degrade when subjected to changes in humidity and temperature. Polyvinyl chloride was applied as a seal layer to such a device in an attempt to improve its durability. The optical switching properties of the device were investigated through an accelerated degradation test conducted at a constant temperature of 30 C and a relative humidity of 80%. Although the device with a seal layer was thicker, it showed superior optical switching properties with a transmittance as high as 54% in the transparent state due to interference at the multilayer. Furthermore, the device with a seal layer exhibited higher durability after 35 days in an accelerated degradation environment, as well as higher switching durability. (C) 2011 Elsevier B.V. All rights reserved.