The states of water in poly(N-vinylpyrrolidone-methylmethacrylate) and poly(N-vinylpyr-rolidone-2-hydroxyethyl methacrylate) hydrogels have been studied by means of DSC．The effect of polymer structure on equilibrium swelling extent of water has been examined．It was found that the state of water and the water content in different hydrogels were dependent on the kind of monomer used，the component of copolymer，the sequence distribution of the monomer unit and the degree of crosslinking (including chemical and physical) to a great extent．The equilibrium water content of the hydrogel may be regarded as the sum of the different state of swollen water contents of the hydroge．Based on these，a semiempirical formula used to estimate the equili-brium water content of the copolymer hydrogels was presented．