The complexation of octyl-(phenyl)-N,N-diisobutyl carbamoyl methylene phosphine oxide (CMPO), a well-known ligand for actinide partitioning, with trivalent actinides and lanthanides was investigated in a room temperature ionic liquid, viz. 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide ([C₄mim][Tf₂N]). Solvent extraction studies in [C₄mim][Tf₂N] medium indicated the participation of three CMPO molecules in the extracted complex. Optical spectroscopic studies in [C₄mim][Tf₂N] confirmed that CMPO forms predominantly ML, ML₂ and ML₃ (where M = Nd³⁺, L = CMPO) types of complexes. The stability constants of the Nd³⁺/CMPO complexes were several orders of magnitude higher in [C₄mim][Tf₂N] than those observed in acetonitrile or methanol, which explains the higher extraction ability of CMPO for trivalent f-cations in [C₄mim][Tf₂N]. Luminescence spectroscopic investigations in [C₄mim][Tf₂N] also predicted the nature of the extracted species as ML₃ (where, M = Eu³⁺, L = CMPO). DFT calculations indicated the formation of ML³₃⁺ as the most populated species, which was further confirmed .by the EXAFS measurements.