Mg-Ni thin films as a switchable mirror material are promising for application in energy-saving windows because they can change their state between reflective and transparent by hydrogenation and dehydrogenation. In our previous work, we found that electrochromic switchable mirror glass based on a Mg(4)Ni thin film is affected by environmental factors such as temperature and humidity. In this work, we investigated the effects of the environment on the optical switching properties of electrochromic switchable mirrors with Mg-Ni thin films of various compositions suitable for a broad range of applications and operating environments. When the mirror devices were kept in a simulated environment with a constant temperature 40 degrees C and a constant relative humidity of 80%, controlled by a thermostat/humidistat bath, their optical switching properties degraded. The degradation was found to be related to the change in the Mg-Ni thin films into nonmetallic states of oxides and hydroxides. The device with a Mg(6)Ni thin film kept in the bath for 7 days showed no optical switching property. The mechanism of degradation in the bath was strongly affected by the composition of the Mg-Ni thin-film optical switching layer. The device constructed with an optical switching layer having high magnesium content degraded more rapidly in the test. (C) 2011 Elsevier B.V. All rights reserved.