The partitioning behavior of two model proteins in poly(ethylene glycol) (PEG)-poly(acrylic acid) (PAA) aqueous two-phase systems has been studied. The effect of polymer molecular weight, tie line length (TLL), pH, temperature and NaCl concentration on the partitioning of proteins (ovalbumin and myoglobin) of different molecular weights and isoelectric points has been studied. Poly(ethylene glycol) is enriched in the top phase while poly(acrylic acid) is found in the bottom phase. An increased partitioning of proteins to the PEG phase has been observed with increased TLL The protein partitioning decreases on increasing the molecular weight of PEG and temperature, whereas the partitioning increases with increase in pH and NaCl concentration of the system. The protein partitioning is better for both the model proteins using PEG 4000-PAA systems at 20 degrees C, pH 8.0 with I M NaCl. The percentage yield of extraction for myoglobin and ovalburnin is 95.2% and 87.4%, respectively. (C) 2008 Elsevier Ltd. All rights reserved.