This paper reports a study of the extraction of copper from ammonia/ammonium sulfate medium with LIX 54, a beta-diketone. The extraction of copper is very sensitive to the equilibrium pH. The highest extraction efficiency is attained in the range of pH 8-9.5. Above pH 10, the percentage of metal ion extracted decreases sharply. The extraction of copper by LIX 54 can be described by the following equation: Cu(NH3)(4,aq)(2+) + 2HR(org) reversible arrow CuR2,org + 2NH(3,aq) + 2NH(4,aq)(+) The thermodynamic equilibrium constant was evaluated in terms of the aqueous phase and was found to be 10((2.89 +/- 0.03)) M-2 at 298 K. The nonideality of the aqueous phase was taken into account by applying the Pitzer model and the extended Debye-Huckel equation. The isotherms of extraction for the system copper/ammonia/ammonium sulfate/LIX 54 revealed that the increase in the salt concentration from 0.2 to 1 M makes the extraction process difficult.