The synergistic action of tri-n-octylamine (TOA) in the selective extraction of the rare earths La, Nd, Tm, Lu, and Y with thenoyltrifluoroacetone (HTTA) was studied in a nitrate media(0.6 M HNO3 + 11.6 g/L CaO). The synergism enhances the REE extraction and separation. The [HTTA], [TOA], diluent, complexing agents, and calcium ion effects were also studied.