The structure and stoichiometry of the lanthanide(III) (Ln) complexes with the ligand 2-thenoyltrifluoroacetone (Htta) formed in a biphasic aqueous room-temperature ionic liquid system have been studied by complementary physicochemical methods. Equilibrium thermodynamics, optical absorption and luminescence spectroscopies, high-energy X-ray scattering, EXAFS, and molecular dynamics simulations all support the formation of anionic Nd(tta)(4)(-) or Eu(tta)(4)(-) complexes with no water coordinated to the metal center in 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide (C(4)mim(+)Tf(2)N(-)), rather than the hydrated, neutral complexes, M(tta)(3)(H2O)(n) (n = 2 or 3), that form in nonpolar molecular solvents, such as xylene or chloroform. The presence of anionic lanthanide complexes in C(4)mim(+)Tf(2)N(-) is made possible by the exchange of the ionic liquid anions into the aqueous phase for the lanthanide complex. The resulting complexes in the ionic liquid phase should be thought of as weak C-4,mim(+)Ln(tta)(4)(-) ion pairs which exert little influence on the structure of the ionic liquid phase.