Thin films were produced by sputtering of metallic Ni in Ar/O-2 and Ar/O-2/H-2 atmospheres. Systematic studies of these films were carried out in electrochromic devices with tungsten oxide and either proton or lithium conducting electrolytes. No major difference was found between devices using nickel oxide films made in the presence or absence of hydrogen. A practically important result is that both kinds of nickel films can be laminated in devices with tungsten oxide without precycling any of the films in a liquid electrolyte. Devices with the used Li+ and H+ conducting electrolytes have similar properties, suggesting that all of them may be relying on proton transport, presumably originating from adsorbed water. From a theoretical point of view, attention is drawn to the fact that the tap of the NiO valence band consists of Ni 3d electron states, in contrast to the O 2p states at hand for most other electrochromic oxides. The anodic electrochromism of nickel oxide may be explained by assuming that the 3d electrons can be reversibly extracted from and inserted into the film without affecting the metal-oxygen bonds.