The phase separation processes occurring in polyurethane/DMSO/water mixtures were studied using DSC and cloud point measurements. It is demonstrated that liquid-liquid demixing occurs in ternary solutions of segmented polyesterurethanes at sufficiently high water concentrations. It is also shown that the hard segment can crystallize from solution when cooled to room temperature; while if the mixture is cooled to sufficiently low temperatures, DMSO partially freezes, which also induces crystallization of the soft segment. (C) 2005 Wiley Periodicals, Inc.