The compositions of coexisting phases have been determined for aqueous two-phase systems containing poly(ethylene glycol) of nominal molecular weight 20000 and copper sulfate at (290.15, 299.15, 308.15, and 317.15) K. The effect of temperature on the liquid-liquid equilibrium was discussed. The experimental compositions of coexisting phases of the systems were used to obtain interaction parameters in universal quasichemical theory (UNIQUAC) activity coefficient models. The experimental tie line data were compared with those obtained using the UNIQUAC model. The results of comparison were indicated by calculation of the root-mean-square deviations (rmsd). The calculated results show good agreement with the experimental data.