[en] The non-stoichiometric nickel oxide (NiO_x, x > 1) films deposited on a conducting substrate by RF magnetron sputtering have been investigated for electrochromism in alkaline solution. It was found that both Ni³⁺ and Ni²⁺ existed in NiO_x films. The intercalation and deintercalation of H⁺ ions causes the bleaching and coloring of NiO_x films. The atomic ratio of Ni and O in the NiO_x films is 0.7:1 (as-deposited); 0.51:1 (bleached); 0.45:1 (colored). The maximal optical density change was found to be 0.78 at a spectral of 326 nm. With the increase of temperature of heat treatment, the electrochromic properties of NiO_x films were weakened