Nickel oxide films, formed by reactive sputtering from a metallic nickel target in argonoxygen atmospheres using an rf diode configuration, show good electrochromic properties when produced at low power densities and high pressures. These results contrast with poorer electrochromic properties observed for films produced under conditions of higher power densities and lower pressures. The optimum conditions result in neutral, grey nickel oxide films which are readily bleached and completely reversible. When deposited on Sn02:F coated glass these films can have a solar transmittance of 0.70 in the bleached state and 0.30 in the coloured state. TEM studies have identified the films to be composed of cubic NiO with grain sizes in the range 2-5 nm.