A hydrogel was prepared by cross-linking poly(4-vinyl phenol) with ethylene glycol diglycidyl ether in order to examine the ion-specific swelling behavior. The swelling ratios to various kinds of aqueous salt solutions except for N(CH3)(4)Cl were almost constant, or the gels remained in a swollen state, up to their saturated concentrations. This unexpected result was ascribed to stabilization of hydrogen-bonding hydrations to the phenol OH proton and to the phenol ring (pi electrons) by anions and cations, respectively. The supposed mechanism was supported by ab initio calculations for the corresponding small model systems.