Fluorine, chlorine, and methane/hydrogen plasma chemistries were investigated for dry etching of thin film MgO, intended for gate dielectric formation on GaN. The chlorine-based chemistry produced much higher etch rates than the other two mixtures, consistent with the relative volatility of the expected etch products. The surface roughened only slightly upon etching with all three chemistries. Small (similar to 1.6 at.%) concentrations of chlorine-containing residues were identified on the MgO surface after Cl-2/Ar etching. Highly anisotropic features were formed in the MgO under these conditions, with good selectivity (similar to7) to conventional photoresist masks. The etch selectivity for MgO over GaN ranged from 0.7 to 5 in Cl-2/Ar discharges. depending on the plasma source power.