A molecular modelling method is used as an aid to predict and to account for the results of solvent extraction of dianions by an ion-exchange extractants such as diphosphonium compounds (polymethylenebis(trioctylphosphonium)s abbreviated as CnBP2+). The extraction ability of CnBP2+extractants was surprisingly dependent on the dianionic species and on the length of bridging methylene chain connecting the two phosphonium cationic centers which offers an effective molecular recognition field. The dianions studied are phthalate, isophthalate, terephthalate, dibenzoyltartrate and zinc oxalate complex. All of them have their negative charges carried by oxygen atoms. The extracted species considered for a dianion A2−are 1:1 and 2:2 ion-pair species such as (CnBP2+, A2−), (2 CnBP2+, 2 A2−rpar; with n = 2, 4, 6, 8, 10, 12. These ion-pair species are modelled and minimized. The total energy values obtained are processed to calculate the association energies of ion-pair species which allow comparison of 1:1 and 2:2 ion-pair species with the assumption that the ion-pair species with the smallest value of association energy should correspond to the most stable extracted species. It results that for phthalate isomers 1:1 association is the most favourable for n = 2, 4 and principally 2:2 association for n≥6. Graphical relationships have been obtained between total and association energies of modelled ion-pair species and experimental extraction percentages of extractable species taken from the literature. The association energy values of ion -pair species with phthalate isomers are compared with those of ion-pair species with dibenzoyltartrate and zinc oxalate complex previously studied.