The quick-scanning extended X-ray absorption fine structure technique of X-ray absorption spectroscopy has been employed to monitor structural changes during the cyclic electrochemical oxidation and reduction of nickel oxide thin film electrodes. The films were prepared by anodic deposition onto a graphite substrate at 1.1 V vs. SCE from a solution of 0.05 M NiSO4 + 0.5 CH3COONa. The freshly "as-deposited" film contained mainly Ni2+ and Ni4+ with highly disordered structures. The phase containing Ni2+ can be charged in 1 M of KOH solution to a phase containing Ni4+. Electrochemical reduction of a charged nickel oxide film at constant current leads to the formation of a Ni(OH), which transforms to P-Ni(OH), with time in the KOH solution. The discharged film can be recharged to form a phase which contains mostly Ni4+. The fraction of Ni4+ in the film increased with charging time and reached a value of 0.8 (i.e., 80% of Ni is present as Ni4+) after 38 min of charging at a constant current of 1.68 mA/cm(2). The Ni4+ phase is structurally similar to that of a NiO2 phase produced by electrochemical oxidation of LiNiO2 in a nonaqueous electrolyte as has been shown previously.