The effect of tri-n-butyl phosphate (TBP) and iso-decanol as phase modifiers on the complexation of Eu³⁺ with octyl-phenyl N,N-diisobutyl carbamoyl methyl phosphine oxide (CMPO) was studied. On an increase of iso-decanol the extraction efficiency of Eu³⁺ decreases. The metal–ligand stoichiometry was found to be 1:3. The Eu complex of CMPO–TBP was more symmetric compared to CMPO–isodecanol. The Eu³⁺ complex exhibited D_3h and C_3h symmetry for 30% and 5% iso-decanol, while that for TBP was C_6v. The radiative, non-radiative life time, electric and magnetic dipole transition probabilities, branching ratio, quantum efficiency etc. were evaluated. The covalency between Eu³⁺ and CMPO is as follows: CMPO–TBP < CMPO-5% iso-decanol < CMPO-30% iso-decanol.