Abstract The extraction behavior of Gd(III) from acidic nitrate-acetato medium by bis(2,4,4-trimethylpentyl)phosphinic acid (BTMPPA, H 2 A 2 ) in toluene either alone or in combination with trioctylphospine oxide (TOPO, B) has been investigated as a function of contact time, concentrations of Gd 3+ , H + , NO 3 − and Ac − (acetate) in the aqueous phase, the concentration of H 2 A 2 and TOPO in the organic phase and temperature. Dependencies of H + , BTMPPA, TOPO and Ac − concentrations on extraction ratio suggest that the extracted species are [Gd(Ac)(HA 2 ) 2 ·H 2 A 2 ] and [Gd(Ac)A 2 ·HA·2B] for without and with TOPO, respectively at lower loading, while GdA 3 is extracted at higher loading in both cases. The synergistic effect of TOPO is observed only at higher TOPO concentrations region. The equilibrium constants are evaluated to be 10 −0.63 and 10 −2.79 for the system without and with TOPO, respectively. The La(III)/Gd(III) separation factors are found to be 348 and 393 for the system without and with TOPO, respectively. The temperature dependence of the extraction data from acidic nitrate-acetato gives the enthalpy of extraction, 56 kJ/mol. The loading capacity for 100 g BTMPPA is found to be 17.89 g Gd(III).