Some of thermodynamic parameters for poly(lactic acid) (PLA)/dialkyl phthalate systems have been investigated. Both poly(DL-lactic acid) (PDLLA) and poly(L-lactic acid) (PLLA), which are stereoisomers of PLA, were used and a series of 1,2-dialkyl phthalates with different alkyl chain, from methyl to hexyl, was adopted as a solvent. Theta temperatures of PLA/dialkyl phthalate system were determined and subsequently the second virial coefficient and the interaction parameter were evaluated. Theta temperature was determined by the extrapolation of the highest liquid-liquid phase separation temperatures and Zimm plots were constructed by static light scattering to obtain second virial coefficient and z-average radius of gyration. Second virial coefficient and z-average radius of gyration was examined quantitatively as a function of temperature and solvent. Thermodynamic parameters that could not be obtained experimentally were calculated based on the Flory-Huggins theory. (C) 2004 Elsevier Ltd. All rights reserved.