Electrochromic (EC) properties of sputtered Ni oxide films have been examined in KCl + H2SO4 acidic aqueous solutions. KCl concentration was kept constant at 1 M while H2SO4 concentrations were varied from 0 to 50 mM. EC coloration efficiency was found to be similar to 30 cm(2)/C in all the solutions, and maximum charge capacity and maximum change in optical density were obtained in a 1 M KCl solution with 0.5 mM H2SO4. Intercalation of hydrogen ions into NiO grains is believed to be the reason for the improvement of charge capacity and optical density change. These results offer support for the practical construction of efficient complementary EC devices using dilute acidic aqueous electrolytes. (c) 2006 The Electrochemical Society.