NiO is a promising alternative to p-GaN as a hole injection layer for normally-off lateral transistors or low on-resistance vertical heterojunction rectifiers. The valence band offsets of sputtered NiO on c-plane, vertical geometry homoepitaxial GaN structures were measured by x-ray photoelectron spectroscopy as a function of annealing temperatures to 600 °C. This allowed determination of the band alignment from the measured bandgap of NiO. This alignment was type II, staggered gap for both as-deposited and annealed samples. For as-deposited heterojunction, ΔE_V = 2.89 eV and ΔE_C = −2.39 eV, while for all the annealed samples, ΔE_V values were in the range of 3.2–3.4 eV and ΔE_C values were in the range of −(2.87–3.05) eV. The bandgap of NiO was reduced from 3.90 eV as-deposited to 3.72 eV after 600 °C annealing, which accounts for much of the absolute change in ΔE_V − ΔE_C. At least some of the spread in reported band offsets for the NiO/GaN system may arise from differences in their thermal history.