Tri-n-butyl phosphate (TBP) reacts with nitric acid to form a hydrogen-bonded complex TBP(HNO3)(x)(H2O)(y) that is highly soluble in supercritical fluid carbon dioxide (SF-CO2). The value of x can be up to 2.5, whereas the value of y varies between 0.4 and 0.8 determined by acid-base and Karl Fischer titrations. The protons of HNO3 and H2O in the complex undergo rapid exchange and exhibit a singlet resonance peak in NMR spectra. When the complex is dissolved in a low dielectric constant solvent, small droplets of nitric acid are formed that can be detected by NMR. Phase behavior studies indicate that the complex forms a single phase with SF-CO2 above a certain pressure for a given temperature. This CO2-soluble Lewis acid-base complex provides a method of introducing nitric acid in SF-CO2 for effective dissolution of uranium dioxide, lanthanide oxides, and perhaps other metal oxides.