We investigated the volume phase transition temperatures of N-isopropylmethacrylamide (NIPMA)-co-N-isopropylacrylamide (NIPA) gels with various mole ratios. Nanometer-sized NIPA-co-NIPMA gel particles were prepared by precipitation polymerization and their swelling behaviors were measured using photon correlation spectroscopy (PCS). After applying the interaction energies found using the molecular simulation technique, we predicted the swelling equilibria for the hydrogels in a water system. For studying crosslinked hydrogel particles in water, we have combined the modified double-lattice theory with Flory and Erman's theory of elasticity. To correct for the deviation of the volume transition temperature, we considered the additional energy parameter(epsilon(h)) between the cross-linker and solvent molecules. We used the corrected model to describe the swelling behavior of the hydrogel and the volume transition temperature. The corrected model was agreed well with their experimental data with no fitting parameters. (C) 2009 Elsevier Ltd. All rights reserved.