A polymer bipolar ion-exchange membrane consists of a layered structure involving one cation and one anion ion-exchange layer joined together in series. In this study, the ionic selectivity and water dissociation rate of six commercial bipolar membranes was evaluated from the measurements of the membrane potential in a concentration cell and the current-voltage curve in a four-point measuring cell. Bipolar membrane technology requires polymer membranes presenting high ion selectivities and water dissociation rates, and in this paper we have addressed the basic physico-chemical phenomena involved, both theoretically and experimentally. We have shown that the effects of the bipolar junction and the membrane fixed charge concentrations on the ion transport rates observed can be understood on the basis of simple concepts.