Equilibrium data of aqueous two-phase systems composed of polyethylene glycol (4000 g mol(-1) or 6000 g mol(-1)) and Li2SO4, (NH4)(2)SO4 or Na2SO4 at pH 6.5 and 25 degrees C were obtained. The efficiency of these in the partition of amylases derived from Aspergillus niger was determined. The experimental data of binodal curves and tie lines were used to estimate the group interaction parameters using the UNIFAC model. Additionally, the influence of phases on the activity of the enzymes was investigated. The results indicate that the polymer molar mass did not influence the biphasic region size. However, the cations under study presented differences in induction to phase formation. It was verified that the systems formed with the Na+ presented a larger biphasic region. The increase in the molar mass of the polymer caused the increase in the exclusion volume from 3970.732 g mol(-1) to 5700.873 g mol(-1). The transfer Gibbs free energy of enzymes presented values between -1296.30 kJ mol(-1) and - 2867.70 kJ mol(-1), that is, the process was spontaneous for all systems studied. The systems formed by (NH4)(2)SO4 and PEG 4000 g mol(-1) presented the best K-e result (3.421) and theoretical recovery of 80.35 %.