Thermally induced phase separation in poly(lactic acid)/dialkyl phthalate systems was investigated. Poly(DL-lactic acid) (PDLLA) and poly(L-lactic acid) (PLLA) with different molecular weights were used. A series of dialkyl phthalates, with different numbers of carbon atoms in the alkyl chain, were employed as solvents to control the interaction between polymer and solvent. The liquid-liquid phase-separation temperature of the poly(lactic acid) solutions decreased systematically with a shorter alkyl chain in the phthalate. Based on the interaction between polymer and solvent and the molecular weight of polymer influencing liquid-liquid phase-separation temperature significantly but crystallization temperature only slightly, proper thermal conditions were employed to investigate competitive phase separation and crystallization in PLLA solutions. Factors that can influence the final morphology of PLLA solutions were examined. (C) 2003 Wiley Periodicals, Inc.