Surface photovoltage measurements of polycrystalline powder samples of nickel oxide have revealed differences in band bending (Vs) related to the defect concentrations of the oxide. The powders prepared at 700°C and 1450°C in air consist of small single crystals with low concentrations ( <109 cm-2) of extended defects, but considerably different nickel vacancy concentrations. The defective NiO (700°C) gave Vs = 760 mV and the equilibrated NiO (1450°C) Vs = 230 mV under UV illumination, possibly due to Fermi level pinning of the Ni 3d8 band near V′Ni and V″Ni, respectively. These results are consistent with values of Vs (i.e. 630 and 240 mV, respectively) for these two samples obtained previously from XPS measurements of the dependence of surface charging on temperature. A visible transition at ≈ 2.5 eV, giving Δφ = 135 mV, is also found in the equilibrated NiO (1450°C) sample but not in the defective NiO (700°C) sample. The results are discussed in relation to other work on the electronic band structure and surface states of nickel oxide.