The kinetics of the forward and backward extraction of the title process have been investigated using a Lewis cell operated at 3 Hz and flux or (F) -method of data treatment. The dependences of (F) in the forward extraction on [Fe 3þ ], [H 2 A 2 ] (o) , pH, and [HSO 4 2 ] are 1, 0.5, 1, and 21, respectively. The value of the forward extraction rate constant (k f ) has been estimated to be 10 27.37 kmol 3/2 m 27/2 s 21 . The analysis of the experimentally found flux equation gives the following simple equation: F f ¼ 10 0.13 [FeHSO 4 2þ ] [A 2 ], on considering the monomeric model of BTMPPA and the stability constants of Fe(III)-HSO 4 2 complexes. This indicates the following elementary reaction occurring in the aqueous film of the interface as rate determining: [FeHSO 4 ] 2þ þ A 2 ! [FeHSO 4 .A] þ . The very high activation energy of 91 kJ mol 21 supports this chemical reaction step as rate-determining. The negative value of the entropy change of activation (294 J mol 21 K 21 ) indicates that the slow chemical reaction step occurs via the S N 2 mechanism.The backward extraction rate can be expressed by the equation:20.5 . An analysis of this equation leads to the following chemical reaction step as rate-determining:2 . However, the activation energy of 24 kJ mol 21 suggests that the backward extraction process is intermediate controlled with greater contribution of the diffusion of one or the other species as a slow process. The equilibrium constant obtained from the rate study matches well with that obtained from the equilibrium study.