Aluminum oxide has been used widely in the microelectronic industry for dielectric films. Etching kinetics of vacuum sputtered aluminum oxide films by warm phosphoric acid is studied. The etch rate is found to be constant when the concentration and temperature of the etchant are fixed and the films being etched come from the same deposition batch. Etch rates obtained at different etchant concentrations show that over the industrial range of interest, the kinetics are first order in the hydrogen ion concentration which is a limiting form of a more general site model. Experiments of varying the etching temperature lead to the conclusion that the process is reaction controlled.