We have developed an electrochromic switchable mirror that consists of all-solid-state thin films. The device can be switched between reflective and transparent states by applying a voltage. This device can potentially be applied to new energy-saving windows because the reflective state of the mirror can effectively control the solar radiation coming into a room. To realize such practical applications, the effects of environmental factors such as temperature and humidity on the optical switching properties of the device should be investigated in detail. In this work, we evaluated the effects of the environment on the device for various constant temperature and relative humidity conditions, which were controlled by a thermostat/humidistat bath. When the device was stored at 50 degrees C and 80% relative humidity (RH) or only 7 days, its surface became much rougher (R(a) = 21.9 nm) as a result of degradation of the Mg(4)Ni thin film on the surface. Moreover, the degraded device lost its optical switching properties. The device was strongly affected by high temperatures and high relative humidity in the atmosphere, resulting in rapid degradation. To address this problem, a device with a protective surface layer was also fabricated, and its durability was evaluated by the same method. The device with the protective layer was found to retain its optical switching properties under a harsh environment. (C) 2010 Elsevier B.V. All rights reserved.