The extraction of Pu4+, PuO22+, Am3+, and Eu3+ by structurally modified sulphoxides in 1-hexyl-1,4-diaza [2.2.2] bicyclooctanium Bis(trifluoromethylsulfonyl)imide was investigated at 25 +/- 2 degrees C in an aqueous acidic medium. The extraction process followed 'cation exchange' mechanism. The phenyl substituted ligands showed higher extraction efficiency due to steric and electronic factors and followed the trend: Allyl phenyl sulphoxide (APSO) > Benzyl phenyl sulphoxide (BPSO) > Benzyl methyl sulphoxide (BMSO). The extraction process was thermodynamically spontaneous and only one ligand was found to coordinate in the extracted species for all the metal ions. Oxalic acid was found suitable for Pu4+ stripping. PuO22+, Am3+, and Eu3+ were efficiently back extracted by the Na2CO3 solution. APSO-IL system showed excellent radiolytic stability and all the three solvent systems were found highly selective.