We report nickel oxide (NiO) fine nanoparticles with an average diameter of 5.3 +/- 1.3 nm as an electrode material for supercapacitor reactions. The superior pseudocapacitive behavior of NiO nanoparticles was demonstrated in three-electrode half-cells and two-electrode button cells, with a maximum specific capacitance of 243 F/g. In situ X-ray absorption near edge structure spectroscopy was used for the first reported time to study the evolution of the electronic structures of NiO during the charge-transfer reactions. We observed that a predominant change of valence state occurred between 0.35 V and 0.50 V (vs. Hg/HgO). The development of NiO nanomaterials in the present study provides a new approach in fabricating advanced pseudocapacitors by utilizing highly cost-effective transition metal oxide electrode materials. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.025210jes] All rights reserved.