Thin films of Al were deposited under UHV conditions and then exposed to oxygen at low pressures over a range of temperatures. The optical properties (at 5461Å) and the thicknesses of the reaction products were characterized by in situ ellipsometer measurements. The results were compatible with a place exchange model wherein oxygen atoms were incorporated into the top surface of the (111) oriented Al. Certain physical parameters estimated from the ellipsometric findings were in agreement with the general view of the place exchange process. Optical parameters of the surface layer show a gradual shift toward those of Al2O3 with prolonged oxygen exposures. © 1982, The Electrochemical Society, Inc. All rights reserved.