The Bureau of Mines studied the extraction of copper (II) from acidic sulfate solutions using commercial extractants LIX 84, P5100, and PT5050. Mathematical models, consisting of sets of nonlinear mass action and mass balance equations, were solved using a commercial equation-solving program on a personal computer. The models suggest that LIX 84 is a mixture of monomelic and dimeric extractant with an extraction equilibrium constant of 145 and a dimerization constant of 65 L/mol; PT5050 appears to be a monomeric extractant with an extraction equilibrium constant of 53; and P5100 shows more complex characteristics, with a monomeric extraction equilibrium constant of 400, a dimer formation constant of 3.7 L/mol, and a modifier-complex formation constant of 5.8 L/mol. For all models, the relative error of fit was less than 6%.