Luminescence and excitation spectra of R3+-doped thorium oxide (R3+ = Eu3+, Dy3+) are reported and analyzed. ThO2 was synthesized and grown as crystal using the flux technique. Usually, R3+ ions substitute Th4+ as a cubic site, but charge compensation often results in the presence of other centers. Doping concentration also has an important effect on the formation of the crystallographic sites. Using selective dye laser excitation at 12 K and selection rules, site symmetries of Eu3+ and Dy3+ ions in TnO(2) are determined and fluorescence from different centers is isolated. Results show that under conditions investigated, Eu3+ ions have O-h, and C-3v Site symmetry in ThO2 while for Dy3+ only C-3v exists. Lifetimes were measured and discussed. A crystal-field (cf) calculation has been performed on the reduced F-7(J) basis of the ground F-7 term of the Eu3+ ion. The crystal-field parameter set is then used to calculate Dy3+ energy levels in ThO2 as a test of its consistency.