Abstract The solution behaviour of the system poly( N-vinyl caprolactam)/water has been studied. Experiments and theoretical calculations indicate a typical Flory–Huggins (Type I) demixing behaviour with a lower critical solution temperature (LCST). The critical concentration and LCST shift to lower values with increasing molar mass of the polymer. The phenomenological description of such a system predicts a continuous temperature controlled swelling behaviour of the corresponding network. A comparison with literature data confirms this prediction. A calorimetric study of the crystallisation and melting behaviour of water and the concentration dependence of the glass transition temperature, gives no evidence for the formation of a polymer/solvent complex.