The complexation of UO22+ with NO3- has been investigated in the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide by UV-vis spectroscopy at T = 18.5 degrees C. The complexation is evidenced through the appearance Of Four peaks at 425, 438, 453, and 467 nm. EXAFS data indicate that the trinitrato Complex, UO2(NO3)(3)(-), is dominating the speciation for a reagent ratio of [NO3-]/[UO22+] > 3. Assuming three Successive complexation steps, the conditional stability constants are calculated, the individual absorption spectra are derived, and a speciation plot is presented.