Two series of ultrathin high-k samples (mixed layers HfxAlyOz and bilayers HfO2 on Al2O3) prepared by atomic layer deposition were investigated using spectroscopic ellipsometry in the energy range of 0.7–10.0 eV. The (effective) optical gap of both mixed layer and bilayer structures can be tuned by the film composition. The optical gap of mixed layers is linearly dependent on the Hf fraction from 5.77 (±0.02) eV for pure HfO2 to 6.71 (±0.02) eV for pure Al2O3. The effective absorption gap of bilayers measured in reflection geometry is lower than that of mixed layers with a comparable Hf fraction due to the dominant effect of the top layer. An increase in film thickness as well as a decrease in refractive index and gap energy was observed after sample storage for two months in atmosphere. The aging effect is likely due to further oxidation of the oxygen deficient high-k films caused by the oxygen diffusion from air into the films.