The protonation equilibria of ethylenediamine (EDA) in cation-exchange membranes are considered in the case of acidified solutions of EDA. Two methods are compared to calculate the apparent equilibrium constants. The first method involves the conversion of the sorbed EDAH(2)(2+) into EDA by sodium hydroxide followed by an acidic titration. In the second method, the membrane is dipped in a solution of carbamic acid (EDACO(2)(+/-)) and the H+ counterions are quantitatively substituted for EDAH(2)(2+). The titration of the remaining EDACO(2)(+/-) by an HCl solution permits to determine the amount of H+ in the membrane at equilibrium. The second method allows to calculate accurately the apparent equilibrium constant K-1* and the apparent acidity constants K-a1* and K-a2*. The apparent and real constants are compared.