To address the existing gap in the literature of upconversion studies involving thoria, samples of thoria doped with Ho3+-Yb3+, Er3+-Yb3+, and Tm3+-Yb3+ were synthesized following epoxide gel method and were characterized. Fluorite structure of the doped samples was evident for the calcined samples from the corresponding xerogels as noticed in their powder X-ray diffraction patterns. The presence of a sharp band near 460cm(-1) in the Raman spectra of all these samples supported the results from diffraction experiments. Intraconfigurational f-f transitions of Ho3+, Er3+, and Tm3+ were present in the UV-visible absorbance spectra of these samples. Both normal emission and upconversion emission from these samples have been studied. Emissions in all three basic colors were observed in the upconversion emission spectra. Two-photon process was found to be responsible for the upconversion in these systems. Decay time measurements for these emissions were analyzed. This synthetic process was further extended to determine the extent of dissolution of heavier lanthanides and found that up to 50mol% of Ho3+, Er3+ and 60mol% of Tm3+, Yb3+ could be dissolved retaining fluorite structure. Creation of oxygen vacancies for these heavily doped specimens was quite encouraging and it could be useful as solid electrolytes in fuel cells.