The solvent extraction of Eu(III) from an aqueous chloride phase into hydrophobic ionic liquids (ILs), 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides ([C1Cnim+][Tf2N−], n = 4, 6, 8 and 10), by β-diketone 2-thenoyltrifluoroacetone (HTTA) alone and in combination with N,N-diisobutyl-2-(octylphenylphosphoryl)acetamide (CMPO) was investigated. The interaction between the two extractants (acidic/neutral) in deuterochloroform has been studied by 1H, 13C, 19F and 31P NMR spectra and NOESY experiments. 1H and 19F Q-NMR analytical techniques were also applied for determination of [C1C4im+] and [Tf2N−] entities transferred into the aqueous phase. Eu(III) was efficiently extracted by HTTA/[C1Cnim+][Tf2N−] and HTTA/CMPO/[C1Cnim+][Tf2N−] systems but a relatively small synergism was established. A cation extraction mechanism was proposed via the log–log slope analysis method. The effect of the IL cation alkyl chain length on the extraction process was studied as well.