Abstract: In 1950, Ionic Inc. invented ion exchange membrane prepared by condensation polymerization of phenol sulfonic acid and formaldehyde. (Juda and MacRae, 1950). This invention attracted a great deal of attention in the world, and contributed to the succeeding development of membrane synthesis technology. Various preparation methods of ion exchange membranes have been reported so far, however typical commercially available membranes applied for saline water desalination are prepared by copolymerization of styrene and divinylbenzene as represented by figure 1.1 and figure 1.2 to obtain a cation and an anion exchange membrane, respectively. It is hardly possible to know membrane synthesizing methods applied in membrane manufacturing plants because it is the secret of secrets. We only estimate synthesizing methods through practicing examples in patents, which are described below.