NiOx films were made by dc magnetron sputtering of Ni in O 2 at a rate of <0.15 nm s-1. The grain size was ∼10 nm and the crystal structure was cubic. Electrochemical hydration, and ensuing conversion to electrochromic NiOxHy, took place in KOH. Spectrophotometric measurements in the 0.35-2.5-μm range showed that it took >100 color-bleach cycles to establish a fully bleached state. When removed from the KOH electrolyte, a colored NiOxHy film increased its transmittance by <1% per h. The luminous and solar transmittance could be varied between 80% and 20% and between 74% and 29%, respectively.