Reactive ion etching of aluminum oxide has been studied in CHF3 and SF6 plasmas generated by electron cyclotron resonance in conjunction with in situ ellipsometric measurement for thickness variation. Because of the involatility of etch products associated with aluminum, purely chemical reactions cannot desorb etch products at room temperatures, and ion bombardment is essential to etch Al2O3 through chemically enhanced physical sputtering. The higher the oxygen content in a film, the faster the etch rate, resulting from chemical sputtering due to volatile CO molecules in CHF3 plasmas. This dependence on composition is absent in SF6 plasma. The threshold ion energy for physi-chemical sputtering by fluorine-containing species is estimated to be about 20 eV at room temperature, while the threshold for Ar sputtering is 50 eV. In CHF3 plasmas, however, Al2O3 exhibits a larger threshold energy at a lower temperature due to passivating species which inhibit sputtering. These passivating species have a very weak binding energy of roughly 0.1 eV, which has been deduced from a temperature dependence of the threshold energy. A patterned sample always shows vertical profile without undercuts.