The interaction between poly(ethylene glycol) (PEG) and water was studied by differential scanning calorimetry (DSC). The DSC curves of PEG-water systems were classified into three groups according to the difference in molecular weight. The melting peaks of eutectic mixture appeared for PEG with molecular weight higher than 1000. The eutectic point temperature shifted to higher temperatures and the eutectic point composition shifted to lower concentrations of PEG with increasing molecular weight. The maximum hydration number per ethylene glycol (EG) unit was estimated as 1.6, 2.4, and 3.3 for samples with molecular weights 400, 1540, and 70,000, respectively. No thermal change was found in PEG1540-water system for a narrow weight fraction range of 0.585-0.605 for overall measuring temperatures due to perfect supercooling. The glass transition temperature shifted to higher temperatures with increasing molecular weight of PEG. A modified Flory-Huggins equation was used to fit curves for experimental liquidus data in phase diagrams. (C) 2001 John Wiley & Sons, Inc.