The extraction of trivalent rare earths ( RE) from nitrate solutions with di-(2-ethylhexyl) 2-ethylhexyl phosphonate (DEHEHP, B) and synergistic extraction combined with 1-phenyl-3-methyl-4-benzoyl-pyrazolone-5 (HPMBP, HA) were investigated. The extraction distribution ratios demonstrate a distinct "tetra effect," and Y lies between Tb and Dy when DEHEHP is used as a single extractant for RE. According to the corresponding separation factors (SF12) for adjacent pairs of rare earths, it could be concluded that DEHEHP could be employed for the separation of La from the other rare earths, and Y from light rare earths. The present work has also found that mixtures of HPMBP and DEHEHP have an evident synergistic effect for RE(III). Taking Y( III) as an example, a possible synergistic extraction mechanism is proposed. The enhancement of extraction in the binary system can be explained due to the species Y(NO3) (.) A(2) (.) HA (.) B formed. The synergistic enhancement coefficients ( R), extraction constants, formation constants and thermodynamic functions of the reaction were calculated. The extraction of heavy rare earths (HRE = Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+ and Lu3+) has also been investigated using such a binary system and compared with that of Y( III). The results show that the synergistic extraction distribution ratios follow the order: Y3+ < Tb3+ approximate to Dy3+ approximate to Ho3+ < Er3+ < Tm3+ < Yb3+ < Lu3+ The separation factors (SFRE/Y) between Y and HRE were calculated and compared with the single extraction system. The possibility of separating Y(III) from heavy rare earths is discussed according to the separation factors.