This research work presents a simple, original and illustrative experimental method to investigate the concenlration polarization using a cation exchange membrane (CEM). The nature of the overlimiting current (Iov) and the water dissociation (WD) on the CEM are particularly studied. They are still widely discussed nowadays. Our experimental method is based on the comparison of the i-v (current transmembrane voltage) curves of the same counter ion with and without buffer coions. For example we compare Na2SO4 and (NaH2PO 4 -t-Na2HPO4), or K2SO4 and (KH2PO4 + K2HPO4). The role of the mixture hydrogen-dihydrogen phosphate coions is to buffer the solution at a neutral pH. The protons, whatever their origin, do not contribute to the current transport. The distance between the two i-v curves of Na2SO4 and NaT (T = buffer coions) represents then the contn'bution of the protons, issued from the WD, to the current transport. This comparison gives unexpected and interesting results: the superposifion of Na2SO4 and NaT i-v curves shows clearly that the WD occurs on the CEM and the protons contribute to the Iov; the superposition of K2SO4 and KT iv curves shows a larger distance between the 2 curves: the water dissociation is many times higher with K Γ· than with Na+; the superposition of NaT and KT gives identical i-v curves. It is an unexpected result: Na + and K Γ· have the same membrane mobilities if the protons do not contribute to the current transport. The overlimiting current increases with voltage: the WD is not a necessary condition to obtain Iov; the contribution ofH + to Iov is not negligible. It represents more than 20% of the total current. The distance between K2SO4 and KT i-v curves measures this conlribution. This experimental method conlributes strongly to a better understanding of the concentration polarization phenomenon. It is only based on experimental evidences and not on hypothesis, speculations or calculations.