The equilibria for the extraction of nickel(II) and ammonia from ammonium salt solutions by using the extractant LIX84-I (active component, 2-hydroxy-5-nonylacetophenone oxime) dissolved in a non-polar diluent, ShellSol D70, were studied at pH 2–10 and 298 K under different ammonium salt concentrations, kinds of counter anions, aqueous phase pH values, and phase ratios. The distribution constant of the nickel–hydroxyoxime complex was independent of the kind and concentration of ammonium salt, and the logarithm of the extraction constant of nickel linearly increased with the ionic strength of the aqueous phase. By using the initial conditions and a model based on the present results and those of our previous study concerning copper extraction (Mater. Trans., 58 (2017), 1427–1433), we were able to predict the extraction behavior of nickel, copper, and ammonia from copper–nickel mixed ammonium sulfate solutions. The agreement between the predicted and experimental data confirmed the validity of our model.