The thermal behaviour of Poly(ethylene glycol) (PEG)/water systems is the subject of the thesis. Differential Scanning Calorimetry was used to investigate the binary phase diagrams of the PEG/water systems. A new method has been developed that utilizes a special custom-made mixing device to measure mixing enthalpies of the liquid PEG/water systems directly in open measuring cells of Differential Scanning Calorimeters (DSC). The liquidus curves of the water in the binary phase diagram were fit by use of the statistical theory of Flory, Huggins, Staverman und Van Santen in consideration of the mixing enthalpies of the systems. A semi empirical derivation was suggested to fit the liquidus curve and the solidus curve of the PEG in consideration of the solid mixed phase.