Aluminum oxidation by ozone produces an aluminum oxide layer which is superior in its corrosion properties compared to natural oxide, as measured by electrochemical methods. The electrochemically measured impedance of the O-3-grown films is similar to 10 times greater than that of O-2-grown films of equivalent thickness. An enhanced pitting potential is observed for the O-3-grown oxide film. Transmission electron microscopy results show that the pore size of O-3-grown oxide films is considerably smaller than that of O-2-grown films. Transmission electron microscopy electron diffraction studies show that the amorphous O-3-grown films are similar to 4% more dense than the O-2-grown film. The initial sticking coefficient for ozone on atomically clean polycrystalline aluminum is 3.8 times larger than for oxygen at 300 K