The extraction of U(VI) by tributylphosphate in the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide), C4-mimTf2N, has been studied as a function of TBP and HNO3 initial concentrations. Extraction measurements have been completed by UV-vis spectroscopy in order to get insights into the extraction mechanism. The proposed chemical model describes the data through a fit of uranyl distribution ratios, while some other suggestions are unable to do so. In this model, uranyl extraction is proposed to proceed via cation exchange at low initial acidities ([UO2(TBP)n]2+ versus C4-mim+ and H+) and via anion exchange at high HNO3 concentrations ([(UO2(NO3)3(TBP)m]− versus Tf2N−). By contrast to the usual TBP/dodecane organic phase, the IL system does not favor the neutral species UO2(NO3)2(TBP)2, and TBP does not extract nitric acid.