Auger electron spectroscopy and ellipsometry, two of the more sensitive techniques for surface studies, have been used to study, simultaneously, both the elemental chemical composition and optical properties of clean and contaminated Al surfaces under vacuum. This combined system has been used to measure the optical constants of both vapor-deposited Al films and ion-cleaned, high-purity bulk Al. The measured optical constants of the contamination-free bulk Al were n=1.566 and k=7.938 where N=n−ik. The measured extinction coefficient k agrees well with the computed value of 7.885 using the free-electron theory. Ellipsometer measurements were made at a wavelength of 6328 Å and an angle of incidence of 70°. Optical measurements made on contaminated surfaces showed that the presence of very thin oxide films can cause large errors in measuring both n and k for Al. Combined measurements showed a linear relationship between changes in the amplitudes of the Auger peaks associated with both O (513 eV) and aluminum oxide (53 eV) and the measured film thickness. This linear relationship was used to estimate the attenuation length of Al Auger electrons (67 eV) in the metal oxide film as 1.9 Å.