The partitioning behavior of L-methionine has been studied in aqueous two-phase systems of (poly(propylene glycol) + sodium phosphate salts + H(2)O) at different temperatures. The salts used were sodium di-hydrogen phosphate (NaH(2)PO(4)), di-sodium hydrogen phosphate (Na(2)HPO(4)) and tri-sodium phosphate (Na(3)PO(4)). The effects of tie line length, salt type, and temperature on the partition coefficient of this amino acid have been studied. In addition, thermodynamic parameters (Delta H(o), Delta S(o) and Delta G(o)) as a function of temperature were calculated. The results showed that increasing tie line length led to decreasing of the partition coefficient. We also showed that the partition coefficients of the amino acid in the systems containing Na(3)PO(4) are greater than the other two salts. Moreover, it is verified that increasing temperature led to decreasing the partition coefficient. The experimental partition coefficient data are correlated using a modified virial-type model. (C) 2011 Elsevier Ltd. All rights reserved.