We study optical properties of Al2O3 films prepared by various techniques using spectroscopic ellipsometry. The film preparation techniques include conventional pulsed magnetron sputtering in various gas mixtures, high power impulse magnetron sputtering, annealing of as-deposited Al2O3 in an inert atmosphere and annealing of as-deposited Al in air. We focus on the effect of the preparation technique, deposition parameters and annealing temperature on the refractive index, n, and extinction coefficient, k, of stoichiometric Al2O3. At a wavelength of 550 nm we find n of 1.50-1.67 for amorphous deposited Al2O3, 1.65-1.67 for amorphous Al2O3 obtained by Al annealing, 1.46-1.69 for gamma-Al2O3 and decreasing n for Al2O3 annealing temperature increasing up to 890 degrees C. The results facilitate correct interpretation of optical characterization of Al2O3, as well as selection of a preparation technique corresponding to a required Al2O3 structure and properties. (C) 2012 Elsevier B.V. All rights reserved.