Electrochromic switchable mirror glass is expected to be implemented in new energy-saving windows, as it can switch between reflective and transparent states simply through an applied voltage. In its reflective state, it can effectively control the solar radiation coming into a room. However, conventional windows are often 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 the mirror. In accelerated degradation studies, when the mirror was kept in a simulated environment with high temperature and high relative humidity controlled by a thermostat/humidistat bath, its optical switching properties degraded. After being kept for 34 days at a temperature of 30 degrees C and a relative humidity of 80%, the mirror never showed its optical switching property. Moreover surface roughness increased up to R(a)=13.1 nm, due to the degradation of the surface layer of the Mg(4)Ni thin film. The layer changed into a non-metallic state of oxide and hydroxide, as determined by the X-ray photoelectron microscopy. We confirmed that the mirror was significantly affected by the environment, especially by high relative humidity in the atmosphere, which caused rapid degradation of the mirror. (C) 2010 Elsevier B.V. All rights reserved.