We have investigated the role of aluminum oxide films as barriers to thermal oxidation of Co, Ni, Fe, Ni80Fe20, Mn, Ta, Cu and Cr in air. The oxidation of the film is monitored by measuring the electrical resistance following a brief anneal in air. We find that 0.3 and 1 nm Al protect the underlying metal film against thermal oxidation in air at temperatures of a few hundred degrees above the temperature at which the unprotected metal oxidizes. These results suggest that magnetic tunnel junction samples could be annealed in air after the oxidation of Al. The expected benefits of the annealing in air would include the oxidation of any remaining metallic Al, a more uniform Al2O3 thickness, and a sharper metal/Al2O3 interface.