A series of hydrophobically modified poly(acrylic acid) gels (HM-PAA gels) were prepared by radical copolymerization of acrylic acid and a small amount of hydrophobic comonomers, 2-(N-ethyl perfluorooctane-sulfonamido)ethyl methacrylate and lauryl acrylate, respectively, in tert-butanol. The effects of hydrophobic modification on the structure and water state in hydrogels were studied by atomic force microscopy, differential scanning calorimetry, and thermogravimetric analyses. Significant differences in the enthalpy of dissociation of water per molar polymer unit were found between unmodified gel and hydrophobically modified gels (HM gels). Three states of water molecules-free water, nonfreezing water, and freezing bound water-were proven to exist in HM gels. The number of nonfreezing and freezing bound water molecules associated with a polymer unit, n(0) and n(1), increased with the hydrophobe content and the hydrophobicity of the hydrophobes. The roles of these water molecules are discussed in relation to hydrophobic association. (C) 2003 Wiley Periodicals, Inc.