A new 2:1 dicationic complex formed by TEMBP with uranyl ion in acetonitrile and two hydrophobic ILs, [BMIm][NTf2] and [N-4III][NTf2], has been identified with combination of optical spectroscopic and mass spectrometric studies. With excess of TEMBP ligand (L/U > 2.0), the uranyl is completely coordinated by two ligands to form a dicationic complex [UO2(TEMBP)(2)](2+). The UV-vis spectra of [UO2(TEMBP)(2)](2+) in acetonitrile and in the two ILs are similar. The vibronic fine structures in UV-vis spectrum of [UO2(TEMBP)(2)](2+) show characters of tetragonal coordination in the uranyl equatorial plane. The symmetry of proposed structure of [UO2(TEMBP)(2)](2+) is D-2h, and its UV-vis spectrum is tentatively interpreted based on the structural similarity to the well studied [UO2Cl4](2-) complex. The luminescence emission spectrum of [UO2(TEMBP)(2)](2+) shows typical vibronic bands, having a mirror relationship with the 455-500 nm region of the corresponding absorption spectrum. The stoichiometry of [UO2(TEMBP)(2)](2+) is confirmed by electrospray ionization-ion trap mass spectrometry (ESI-ITMS) studies with acetonitrile as solvent. The "naked" dication (m/z 423) is characterized by the remarkable eight peaks with interval of 14 m/z units in its tandem mass spectra, representing the fragmentation of ligands by losing C2H4 units from their ethoxy groups. However, the dication tends to exist as a weak adduct with either in additional ligand or an anion in the ESI mass spectrum. The adducts {[UO2(TEMBP)(2)](2+) + TEMBP} (m/z 567) and {[UO2(TEMBP)(2)](2+) + [ClO4](-)} (m/z 945) are favorable in pure acetonitrile, while only one adduct {[UO2(TEMBP)(2)](2+) + [NTf2](-)} (m/z 1126) is predominant in [BMIm][NTf2] (diluted with acetonitrile). The results of ESI-ITMS study are consistent with those of optical spectroscopic studies.