Thermodynamic behavior and water state in the hydrogels were investigated for the fast responsive thermosensitive poly(N-isopropylacrylamide) hydrogels which were prepared by adding CaCO3 as a pore-foaming agent during the polymerization．The results indicated that the swelling ratio increased as the content of CaCO3 in the reactants increased．The effect of crosslinking agent on the swelling ratio showed that the more the crosslinking agent content

the lower the swelling ratio．The gel phase transition temperature was not influenced by the content of CaCO3 and crosslinking agent

but it decreased linearly as the NaCl content in water solution increased．DSC was used to study the state of water in the porous hydrogels swollen in deionized water and different content of NaCl solution. Results showed that three states of water exist in all these porous hydrogels. The effect of the content of CaCO3 and ionic strength on the three states of water was also investigated．