Extraction of copper from water using LIX 84 extractant was investigated. Binary equilibrium data are reported and predictive models have been developed. With a simple, concentration-based equilibrium model, an equilibrium constant of 3.9 was obtained. This model is adequate when no acid is present in the aqueous phase. As the acidity of the aqueous phase increases, nonideality becomes significant and needs to be considered to predict extraction equilibrium properly. Our second model incorporates activities and major ionic reactions for the aqueous phase species; nonideality of the aqueous phase is successfully modeled. Nonideality of the organic phase is lumped into a concentration-dependent equilibrium constant. Equilibrium constants of 27.2, 13.1, and 6.2 were obtained for 5, 10, and 20 v/v% LIX 84 in tetradecane, respectively. This model can effectively predict the extraction equilibrium over a wide range of pH.