Nickel oxyhydroxide (NiOOH) thin films were prepared by reactive sputtering using H2O as a reactive gas, and the electrochromic (EC) properties of these thin films were investigated in alkaline, neutral, and acidic aqueous electrolyte solutions. A stable EC cycling of up to 100 cycles was obtained for the NiOOH thin films in electrolyte solutions with a wide pH range of 4.1-13.5. However, the EC cycling degraded drastically after several tens of cycles in an electrolyte with pH=2.0. In the stable pH region, the NiOOH films exhibited a large transmittance change from approximately 30-90%, although the response speed slowed with decreasing pH of the electrolytes. (C) 2011 Elsevier B.V. All rights reserved.