The UO22+ and NpO2+ extraction complexes with n-octyl(phenyl)-N,N-diisobutylmethylcarbamoyl phosphine oxide (CMPO) and diphenyl-N,N-diisobutylcarbamoyl phosphine oxide (Ph2CMPO) have been investigated by density functional theory (DFT) in conjunction with relativistic small-core pseudopotentials. For these extraction complexes, especially the complexes of 2:1 (ligand/metal) stoichiometry, UO22+ and NpO2+ predominantly coordinate with the phosphoric oxygen atoms. The CMPO and Ph2CMPO ligands have higher selectivity for UO22+ over NpO2+, and for all of the extraction complexes, the metal-ligand interactions are mainly ionic. In most cases, the complexes with CMPO and Ph2CMPO ligands have comparable metal-ligand binding energies, that is, the substitution of a phenyl ring for the n-octyl group at the phosphoryl group of CMPO has no obvious influence on the extraction of UO22+ and NpO2+. Moreover, hydration energies might play an important role in the extractability of CMPO and Ph2CMPO for these actinyl ions.