Abstract: The pH-sensitive swelling isothems of various acrylic acid hydrogels were mea-sured.Generally ,the Columbic interactions inside the gel network and the Donnan equilibriabetween the network and bulk solution are two main factors affecting the swelling behaviorof pH-sensitively hydrogels.The type and the amount of the cross-linking agent and the con-centration of the monomer used during the preparation of gels can effectively modify the net-work structure,therefore,can change the appearance of awelling isotherms.Besides,the prob-ability of forming non-gaussian short chains andleading to the entanglement of chains isgreatly enhanced with the increase of the amount of the cross-linking agent and the concen-tration of the monomers. As a consequence,the maximum swelling ratio is notably reduced.