Phase transition behaviour of poly(ethylene glycol) (PEG)-water systems was investigated by differential scanning calorimetry (DSC) in a temperature range from 150 to 350K and water content (mass of water/mass of PEG) range from 0 to 10gg(-1). In DSC heating curves, glass transition, cold crystallization, melting of eutectic crystal, water and PEG crystal were observed depending on water content. The cold crystallization of the system, which is thought to be used as an index of biocompatibility of polymer-water interaction, received particular attention. It was found that cold crystallization and glass transition were observed in a wide water content range from 0.05 to 10gg(-1). From the enthalpy balance of transitions in both heating and cooling DSC curves, it was confirmed that cold crystallization is attributable to the molecular rearrangement of PEG molecules associated with amorphous ice. When four water molecules are attached to one repeating unit of PEG, the heat capacity difference at glass transition temperature attains the largest value and the enthalpy of cold crystallization shows the maximum value. (C) 2007 Published by Elsevier B.V.