The swelling behavior of poly(N-isopropylacrylamide) (PNIPA) hydrogels in aqueous solution of poly(ethylene glycol) (PEG) and the partition parameter of PEG between the gel and the external solution were investigated. The molecular weight of PEG, the ionic group content of the hydrogels and the swelling temperature were taken as the independent variables. Several types of volume phase transitions such as swelling, deswelling and reentrant phase transitions were observed depending on the external parameters. The results were explained in terms of the theory of equilibrium swelling.