In polarimetry, it is important to characterize the polarization properties of the instrument itself to disentangle real astrophysical signals from instrumental effects. This article deals with the accurate measurement and modeling of the polarization properties of real aluminum mirrors, as used in astronomical telescopes. Main goals are the characterization of the aluminum oxide layer thickness at different times after evaporation, and its influence on the polarization properties of the mirror. The full polarization properties of an aluminum mirror are measured with Mueller matrix ellipsometry at different incidence angles and wavelengths. The best fit of theoretical Mueller matrices to all measurements simultaneously is obtained by taking into account a model of bulk aluminum with a thin aluminum oxide film on top of it. Full Mueller matrix measurements of a mirror are obtained with an absolute accuracy of ∼1% after calibration. The determined layer thicknesses indicate logarithmic growth in the first few hours after evaporation, but stability at a value of 4.12 ± 0.08 nm in the long term. Although the aluminum oxide layer is established to be thin, it is necessary to consider it to accurately describe the polarization properties.