Electrodialysis employs specially prepared membranes which are semipermeable to ions based on their charge, and it employs electrical current to reduce the ionic content of water in one of the most widely used desalting processes. In this paper we determine the parameters which characterize the working optimum of an electrodialysis reversal pilot plant, i.e., the values of the applied voltage (V) used in the electrical stages and the pressure of the feedwater (P). These values correspond to the maximum separation percentage (%S) and are obtained by plotting %S against V and P.