The oxidation state of nickel in a nickel oxide battery has been examined using x-ray absorption spectroscopy. Chemically synthesized beta-NiOOH and BaNiO3 were used as reference standards for the Ni(III) and Ni(IV) oxidation states of nickel, respectively. The shift of the Ni K-edge absorption in both a completely charged nickel oxide electrode (NOE) and a 1000% overcharged nickel oxide electrode corresponds to a Ni oxidation state of +3.5. This oxidation state clearly indicates the presence of Ni+4 in the charged electrodes.