Anodic oxidation of aluminum results in a mesoporous oxide film. The thin-film geometry of our samples enables straightforward optical modeling of ellipsometry spectra of fully anodized films, using only three physically relevant parameters. The system of randomly distributed, but aligned cylindrical pores gives rise to an optical anisotropy, which is incorporated using the original work of Bruggeman. The optically determined film thickness and cylinder porosity only agree with electron microscopy results when the oxide is considered as a nanoporous matrix. Upon chemical etching, the cylinder porosity increases with time from 10 to 80%, while the aluminum oxide nanoporosity of 33% hardly changes. (C) 2003 The Electrochemical Society.