The partitioning behavior of soluble proteins from tannery wastewater using aqueous two-phase system (ATPS) was investigated. An ATPS polyethylene glycol (PEG)/MgSO4 was examined with regard to the effects of PEG molecular weight (MW) and concentration, MgSO4 concentration, pH and NaCl concentration on protein partition and extraction. The partition coefficients measured for soluble proteins were proportional to the difference in PEG concentration between the phases. The MW and concentration of PEG were found to have significant effects on protein partition and extraction with low MW PEG4000 showing the best conditions for the partitioning of protein in PEG + MgSO4 + water system. Sulfate salt was chosen as the phase-forming salt because of its ability to promote hydrophobic difference between the phases. This system was operated at room temperature (30 degrees C). Increase in pH of the system increases the partition coefficient of proteins from tannery wastewater. The addition of sodium chloride showed significant influence on the partition coefficient. ATPS comprising PEG4000-magnesium sulfate provided a means for the recovery of proteins from tannery wastewater. The maximum percentage yield of protein extracted is 82.68%. (c) 2006 Elsevier Ltd. All rights reserved.