An experimental investigation concerning the dissociation equilibrium in a gel accompanied by swelling and shrinking was carried out by using hydrolyzed polyacrylamide gel and buffer or NaOH solution. It was found that the gel swelled or shrank to absorb or release mobile ions and water, and the degree of change depended on the pH of solutions. In the system of Na-type gel, -COONa, and buffer solution, the relationship between the degree of dissociation of the fixed carboxyl acid in gel and the concentration of hydrogen ions in the outer aqueous phase could be correlated by an equation based on the dissociation equilibrium. In the system of H-type gel, -COOH, and NaOH solution, though the mechanism differed according to pH, the relationships conformed to equations based on the reaction equilibrium. The analytical methods developed here may be expected to apply to the enriching separations of various polymer solutions and metal ions.