The swelling equilibrium of an ionic polymer gel soaked in a solution of two components has been investigated in terms of a modified Flory-Huggins model. A variety of phase diagrams are calculated for gel systems of liquid 1/liquid 2/polymer network for different values of the model parameters. It can be anticipated that gels can change their volume discontinuously even in a mixture of partially miscible liquids. In some systems, there is no critical point though a discontinuous change does take place. For gel systems of solvent/electrolyte/polymer network, calculation shows that the concentration of electrolyte inside the gel will change discontinuously at a point of discontinuous volume change. Calculation of transition points under a uniaxial swelling condition successfully explains the composition hysteresis between swelling and shrinking transitions.