The influence of factors limiting both the purity and the concentration levels of HCl and NaOH produced by electrodialysis of NaCl with bipolar membranes is analysed. The bipolar membrane is the WSI membrane set up by Simons. Co-ion leakages through homopolar and bipolar membranes an measured under different experimental conditions. The co-ion leakages through the homopolar membranes lead to acid and base containing between 1 and 4% weight of salt. The co-ion leakages through homopolar membranes limit the concentration of the products. A theoretical calculation of both the concentrations of the produced acid and base, and also their degree of purity is achieved. This model allows the prediction of the level of concentration of the products as a function of the water transport, the co-ion leakages through homopolar membranes and the current density.