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 by bis-(2, 4, 4-trimethylpentyl)phosphinic acid can be represented by: Ff (kmol m–2 s–1) = 10–8.64 [Cu2+](ini) [HSO4–]1. Analysis of this equation gives, Ff (kmol m–2 s–1) = 10–4.88 [CuHSO4+] [A–]. An activation energy of ca. 13.7 kJ mol–1 indicates that the diffusions of 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 [CuHSO4A.HA] complex in the organic phase by an aqueous sulfate-acetato solution can be expressed as: Fb (kmol m–2 s–1) = 10–5.68 [CuHSO4A.HA](o) (1 + 10–3.67 [H+]–1)–1. Analysis of this equation results in two equations, Fb (kmol m–2 s–1) = 10–9.797 [CuHSO4A.HA](o) [A–]–1 [HSO4–]–1 and Fb (kmol m–2 s–1) = 10–8.127 [CuHSO4A.HA](o) [A–]–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 Kex (rate) (kf/kb = 10–2.967) matches well with the Kex (eq) (10–3.19) value within the experimental limitations.