This study evaluates the influence of various process parameters such as type of aqueous two-phase systems (ATPSs), phase-forming salt, molecular weight of the phase-forming polymer, phase composition, phase volume ratio, type of neutral salts, and temperature on the extraction yield of Zn(II) ions to determine the best conditions for the recovery of Zn(II) ions in ATPSs composed of polyethylene glycol (PEG) and a salt (sodium sulfate, ammonium sulfate, potassium citrate, and sodium citrate). In all the systems, Zn(II) ions presented affinity for the PEG-rich phase. The 31.36% PEG2000+7.45% sodium citrate system with NaCl showed the highest values of extraction yield. In addition, thermodynamic parameters (Delta H-t degrees, Delta S-t degrees, Delta G(t)degrees) as a function of temperature were calculated for this system, and the results implied that the partitioning of Zn(II) ions was controlled by entropy changes.