Electrochromic (EC) properties of sputtered Ni oxide films have been examined in 1 M KCl + H2SO4. acidic aqueous solutions with H2SO4 concentrations of 0-50 mM. EC coloration efficiency comparable to that in alkaline electrolytes was obtained in all the solutions and no remarkable degradation in charge capacity was observed up to 100 cycles. These results offer support for the practical construction of efficient complementary EC devices using dilute acidic aqueous electrolytes. (c) 2007 Elsevier B.V. All rights reserved.