In situ scanning tunneling microscopy (STM) was used to study anodically grown oxide on Ni(100) and on polycrystalline Ni in 1 M NaOH. Oxidation at low potentials (-0.7 to -0.5 V vs NHE) resulted in a well-ordered rhombic structure which has not been previously identified. This structure was resistant to reduction, which suggests that it is intimately linked to the irreversible nature of the Ni/Ni(OH)(2) voltammetric peak. As the potential was increased through the passive regime, the rhombic structure became distorted, and, at higher potentials (>0.18 V vs NHE), a quasi-hexagonal structure was observed with a nearest-neighbor spacing consistent with either beta-Ni(OH)(2)(0001) or NiO(111). In the transpassive regime, similar to 0.6 V, the hexagonal structure was largely unaltered by the Ni(OH)(2)/NiOOH reaction. This is consistent with the proposed one-electron oxidative deprotonation/proton insertion mechanism. However, if the potential sweep was extended to 0.8 V, an additional modulation of the electron density occurred with a characteristic length of 1.5-2.0 nm.