We consider distribution of 1:1 salt between an external reservoir and a mesoscopic selectively swollen ionomer gel. We examine the effect of the gel morphology on the salt concentration in the gel. Applying a molecular thermodynamic model of a diblock copolymer ionic gel [A.I. Victorov, C.J. Radke, J.M. Prausnitz, Phys. Chem. Chem. Phys. 8 (2006) 264] we show that the deviation from the classical Donnan rule may be substantial and depends on the gel morphology. A nonuniform gel takes up more salt than is predicted by the Donnan rule. Strongest deviations are found for weakly and moderately charged gels at low salt and thus these findings have direct biomedical relevance. Results are given for diblock copolymer gels of lamellar, bicontinuous, cylindrical and spherical morphologies over a wide range of solution salinities and ionization degrees of the gel. Combined effects of morphology on equilibrium swelling of the gel and on the deviation from the Donnan rule are illustrated.