Abstract: In this paper, the synergism in the extraction of Europium(Ⅲ) and Gadolinium (Ⅲ) with the mixed extractants of thenoyltrifluoroacetone (HTTA) and trialkylmethylammonium nitrate (N₇₄₀₂) has been reported. The optimun conditions of the HTTA-extraction, N₇₄₀₂-extraction and,HTTA N₇₄₀₂}-synergistic extraction have been determined. The separation factors of europium and gadolinium in these systems have been determined also. The extractable species of europium (Ⅲ) and gadolinium (Ⅲ) from synergislic extraction are ion-associated pairs and their stabilities exhibit the "nverted ordinal phenomenon". From the method of slope-analysis, the compositions of these two extractable species are probably(R₃NCH₃)+·﹝Eu (TTA)₃NO₃﹞- and (R₃NCH₃) + ·﹝Gd(TTA) ₃NO₃)^-. The mechanism of the synergistic extraction in these two systems has been discussed.