Potentiostatic current oscillations occur in electro-oxidation of aluminum alloy 2024 by ammonium peroxydisulfate in phosphoric acid, for which we find nearly periodic behavior at 60 degrees C, and cyclic bursting at 68 degrees C. We also show that potentiostatic current oscillations occur in electro-oxidation of the same alloy by sodium dichromate in sulfuric acid. The results are discussed in terms of the separate roles of the oxidant and acid in oxidation of the alloy and dissolution of the film, respectively, and the apparent requirement that oscillatory electro-oxidation of aluminum requires a balance between oxidant strength and acid strength.