The physico-chemical equilibrium properties of two functionalized cation-exchange membranes as well as their evolutions with temperature have been investigated in this report. An original method for measuring absorbed carbon dioxide (CO2) and apparent equilibrium constants inside these membranes based on a gas-specific electrode has been used. Logarithm of the experimental apparent equilibrium constant values showed inverse proportionality with membrane temperature, We have taken into consideration four combined effects that explain the decrease in the equilibrium constant with temperature based on the enthalpy change, the loss in gel-water content of the ion exchanger with membrane temperatures, the exchange capacity (molality), and the solubility of carbon dioxide inside the membrane, From the Van＇t Hoff plots and assuming constancy in the heat capacity of the membrane, the thermodynamic quantities enthalpy change, entropy change and Gibbs free energy change for the formation of carbamic acid from one mole of gaseous carbon dioxide and two moles of monoprotonated ethylenediamine (reactive counterions) have been estimated for each membrane sample. In a temperature range between 20 and 50 degrees C, it has been shown that this complexation is exothermic with a heal of reaction in the order of magnitude -38 kJ mol(-1) inside Permion(R) 5010 M and -39 kJ mol(-1) inside Nafion(R) 120. The corresponding entropy and free energy changes are equal to -62 J K-1 mol(-1) and 62 x 10(-3) T -38 kJ mol(-1) in Permion(R) and -74 J K-1 mol(-1) and 74 x 10(-3) T - 39 kJ mol(-1) in Nafion(R).