A new tri-n-butylphosphate nitric acid (TBP-HNO3) adduct was prepared by combining TBP and fuming (90%) HNO3. The adduct was characterized, and its phase-equilibrium behavior in supercritical carbon dioxide is reported. Supercritical carbon dioxide (sc-CO2) was modified with this new adduct [TBP(HNO3)(5.2)(H2O)(1.7)], and the extraction efficacies of selected rare earth oxides (Y, Ce, Eu, Tb, and Dy) at 338 K and 34.5 MPa were compared with those obtained using an adduct formed from concentrated (70%) HNO3 and TBP [TBP(HNO3)(1.7)(H2O)(0.6)]. All rare earth oxides tested with both adduct species could be extracted with the exception of cerium oxide. The water and acid concentrations in the different adducts were found to play a significant role in rare earth oxide extraction efficiency.