The synergistic extraction of samarium (Sm(III)) using two different main extractants and three organic phosphine oxides as synergist was examined. A synergistic effect was observed when dibutylmonothiophosphoric acid (DBTPA) was used with tri-n-butylphosphine oxide (TBPO) or dibutylphosphate (DBOBPO). The order of extractability was TBPO > DBOBPO. On the other hand, the extractability of samarium in the DBTPA + tri-n-butylphosphate (TBOPO) system was not significantly greater than the extractability of samarium in the DBTPA system. An antagonistic effect was observed when dibutylphosphoric acid (DBPA) was used as the main extractant and TBPO as the synergist. The extractability of samarium in the DBOBPO/TBOPO system was almost the same as that of samarium in the TBPO system. This difference in the extractabilities observed when using different synergists with the two main extractants probably resulted from the balance between the association abilities of the synergist and the main extractant. This could be explained by the basicity of the oxygen atom in the synergists and the coordination ability of the synergist with the samarium/main extractant complex. This was in turn due to the coordination number of the aforementioned complex.