The kinetics of the forward and backward extractions of copper(II) by the bis(2,4,4-Trimethylpentyl)phosphonic acid process have been investigated using the single drop technique and flux (F)-method of data treatment. The (F)-equation for the forward extraction of Cu(II) from the sufate-acetato medium bybis-(2,4,4-trimethylpentyl)phosphinic acid can be represented by: F-f (kmol m(-2) s(-1)) = 10(-8.64) [Cu2+]((ini)) [H+]((int))(-1) [H(2)A(2)]((o))(0.5) [HSO4-](1). Analysis of this equation gives, F-f (kmol m(-2) s(-1)) = 10(-4.88) [CuHSO4+ and A(-) to the aqueous film of the interface (and not the chemical reaction between these two species to form CuHSO4.A) are rate controlling. The (F)-equation for the backward extraction of Cu(II) from the [CuHSO(4)A.HA] complex in the organic phase by an aqueous sulfate-acetato solution can be expressed as: F-b (kmol m(-2) s(-1)) = 10(-5.68) [CuHSO(4)A.HA]((0)) (1 + 10(-3,67) [H+](-1))(-1) [H(2)A(2)]((o))(-0.5) [SO42-](-1) . Analysis of this equation results in two equations, F-b (kmol m(-2) s(-1)) = 10(-9.797) [CuHSO(4)A.HA]((0)) [A(-)](-1) [HSO4-](-1) and F-b (kmol m(-2 S-)1) = 10(-8.127) [CuHSO(4)A.HA]((o)) [A(-)](-1) [SO42-](-1) at higher and lower aqueous acidities, respectively. Low activation energies (< 20 kJ mol(-1)) indicate that the stripping processes are diffusion controlling at both regions of H+ concentration. The value of K-ex ((rate)) (k(f)/k(b) = 10(-2.967)) matches well with the K-ex ((eq)) (10(-3.19)) value within the experimental limitations.