Electrochromic switchable mirrors are expected to be used in energy-saving windows because their optical properties can be changed by application of a voltage. For practical use, the negative impact of environmental conditions on the optical switching properties of the device should be minimized. In this work, the durability of an electrochromic device was investigated in a thermostat/humidistat chamber at -5 degrees C. After devices were stored at sub-zero temperatures, the switching speed and maximum transmittance decreased only slightly with increasing exposure time. In other words, degradation of the device did not significantly occur at sub-zero temperatures. In comparison, the optical switching properties of a device stored at 40 degrees C and 60% relative humidity severely deteriorated. This deterioration was attributed to the degradation of the surface of the Mg4Ni optical switching layer. This layer was affected by the environmental conditions, and in particular changed from the metallic magnesium state to a non-metallic state. In future work, we will investigate suitable measures for preventing environmental damage to the device. (C) 2012 Elsevier B.V. All rights reserved.