Tantalum pentoxide (Ta2O5) thin films have been deposited by reactive ion beam sputtering at room temperature. The films have been characterized by scanning electron microscopy, atomic force microscopy, x-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), variable angle ellipsometry, and UV-VIS spectroscopy. The main parameter varied was the oxygen partial pressure. Stoichiometry is reached for p(O) >= 3 x 10(-2) Pa; within the limit of XPS (similar to 1%) the films are free of contaminations. They are extremely smooth with a surface roughness of 0.14 nm only. From FTIR, it can be concluded that they are amorphous. For stoichiometric Ta2O5 films, the refractive index at 532 nm is in the range from 2.05 to 2.2, while the extinction coefficient is below the detection limit of our ellipsometer. UV-VIS spectra show stoichiometric films to possess a high transmission in a wide wavelength range with an absorption edge below 300 nm. (C) 2008 American Vacuum Society.