The synergistic solvent extraction behavior of Nd(III) with HSTTA and the neutral donors dipyridyl, tributylphosphate (TBP) and tri- n-octylphosphine oxide (TOPO) (≡L) have been investigated using benzene as the organic solvent and an ionic strength 0.1 (NaClO 4). The extraction process is governed by the following reaction equilibrium Nd 3+ 3HSTTA (O)+n L (O)⇆ Nd(STTA) 3 (L) n(O)+ 3H + The extraction constants ( K 3,n ∗ ) and the adduct formation constants ( β 3, n ) have been evaluated. With dipyridyl, only mono adduct formation takes place, whereas, with TBP and TOPO, both mono and bis adducts are formed. The β 3, n values for HSTTA systems are higher than for the corresponding HTTA systems.