Abstract A method of characterization of the protonation equilibrium of ethylenediamine (EDA) in a cation-exchange membrane is propounded. This method involves an intermediate reaction of the membrane (initially in the mixed EDAH +EDAH 2 2+ form) with gaseous carbon dioxide then the elution and the titration of the carbamic acid formed. This method is more precise and is preferred to the conversion of the sorbed EDAH + and EDAH 2 2+ into EDA by sodium hydroxide followed by an acidic titration. The selected method allows to calculate accurately the apparent equilibrium constant K ∗ of the protonation of EDA in the membrane and is very useful to characterize the carrier (EDAH +) content of membranes conditioned to achieve facilitated transport of carbon dioxide. The data obtained will be used in an improved analysis of this transport process.