Transparent nanostructured NiO electrodes have been prepared by heating Ni(OH)(2) sol-gel films at a temperature of 300-320 degreesC. Nanostructured NiO (bunsenite) behaves as a p-type semiconductor and has an indirect band gap of 3.55 eV. It shows a strong anodic electrochromic effect, as it changes color from transparent to brown-black upon application of positive potentials. This effect is caused by oxidation of Ni atoms located at the NiO/electrolyte interface. Electrochemical oxidation reactions are highly reversible in both aqueous and nonaqueous electrolytes. In aqueous electrolyte, the half-potentials show a Nernstian pH dependence, whereas in nonaqueous electrolytes, the type of cation present determines the shape and position of the cyclic voltammogram.