Electron-beam-evaporated nickel oxide films were shown to have a microcrystalline cubic nickel oxide structure. The pressure in the chamber during the film deposition has a large effect on the crystal size and the stoichiometry of the films. The redox currents, the efficiency of these films to color, and their optical properties were influenced by the processing conditions employed and the resulting microstucture. A model is proposed based on the observed microstructure that explains the source of overstoichiometric oxygen and ion transport in these films.