MgO and Sc 2O 3 were deposited by gas source molecular beam epitaxy on GaN. MgO was found to produce lower interface state densities than Sc 2O 3, 2-3 × 10 11 vs. 9-11 × 10 11 eV -1 cm -2. The good electrical quality of the interface is believed to be due to the presence of a single crystal epitaxial layer at the GaN surface. By contrast, the MgO was found to be more sensitive to environmental and thermal degradation than the Sc 2O 3. The environmental degradation is believed to be due to interaction with water vapor in the air and was suppressed by capping of the MgO. Annealing at the temperatures needed for implant activation in GaN produced significant roughening of the MgO/GaN interface and an order of magnitude increase in the interface state density. This sensitivity to thermal degradation will require changes in the processing sequence presently envisioned for e-mode devices in order to avoid damaging the interface and increasing the gate leakage in the device.