The paper deals with physicochemical investigations of anionic membranes in view to better understanding the proton leakage phenomenon in the treatment of acidic solutions. The membranes of interest consisted of poly (4-vinylpyridine) ionisable groups grafted on two different copolymer structures. The acidity constant of the two membranes has been determined by confocal Raman microscopy, depending on the pH of the equilibrating acidic solutions, and the nature of the acid tested. In addition the membranes were shown to exhibit an appreciable electrical conductivity and water content only in the protonated form: the values obtained can be correlated with the pK(a) of the ionisable groups. Calculations of the chemical composition of the membrane in equilibrium with the acidic solutions revealed that the aqueous phase inside the membrane is far less acidic than the external equilibrating solution, leading to the acidity constants in the membrane ((pK(a)) over bar (true)) and another related to the solution phase (pK(a)(apparent)). This fundamental work led to the concentration range of the acidic solution to be treated: the pH of the acidic solution has to be of the order of the apparent pK(a) of the membrane for appreciable content of active poly (4-vinylpydinium) groups and high selectivity of the ion transport through the membrane