We used spectroscopic ellipsometry to determine the optical constants of seven ThO2 thin-film samples, thickness ranging between 28 and 578 nm, for the spectral range of 1.2 to 6.5 eV. The samples were deposited by biased radio-frequency sputtering at DC bias voltages between 0 and - 68 V. The index of refraction, it, does not depend on bias voltage, sputter pressure, deposition rate, or thickness. Specifically, the value of 11 at 3 eV is 1.86 +/- 0.04 for the unbiased samples and 1.86 +/- 0.04 for the biased samples. The average value of n at 3 eV for the thicker samples (d >= 50 nm) was 1.87 +/- 0.05, and 1.85 +/- 0.02 for the thinner samples (d <= 50 nm). (c) 2006 Elsevier B.V. All rights reserved.