The equilibrium constant values of copper(II) extraction from phosphoric acid media by LIX 622 (67.7% 5-dodecylsalicylaldoxime, Cognis Ireland) taking into account the formation in the organic phase of the complex CuL2, HL being 5-dodecylsalicylaldoxinie, at different ionic strengths have been calculated. Elmore's model has been considered for the speciation of the aqueous phase. A new approach for the calculation of the activity coefficient of H2PO4- has also been proposed. The extraction equilibrium constant values obtained at the different ionic strengths have been successfully correlated considering the activity coefficients of the species in solution as well as the release of water molecules in the extraction process. The extraction constant value at infinite dilution of the complex CuL2 is also reported. (C) 2009 Elsevier B.V. All rights reserved.