We use spectroscopic ellipsometry to determine the optical constants of seven thin-film ThO2 samples deposited by radio-frequency sputtering, thickness ranging between 24 and 578 nm, for the spectral range of 1.2 to 6.5. We used a hollow-cathode light source and vacuum monochromator to measure constants at 10.2 eV. None of the deposition parameters studied including DC-bias voltages successfully increased the n of (that is, densify) thoria films. The value of n at 3.0 eV is 1.86 ± 0.04. We find compelling evidence to conclude that the direct bad gap is at ~5.9 eV, clarifying the results of others, some of whom observed the absorption edge below 4 eV. the edge in the two thickest films is of a narrow feature (FWHM = 0.4 eV) with modest absorption (α~ 6µm-1, K~0.1). Absorption may go down briefly with increasing energy from 6.2 to 6.5 eV). But at 10.2 eV absorption is very high and index low as measured by variable angle reflectometry, α= 47.3 ± 5.5 µm-1 and K= 0.48 ± 0.05, and n= 0.87 ± 0.12.