Abstract: There are some problems if adopting conventional theories such as the double electric layer and the Donnan equilibrium to explain the permselectivity of ion exchange membranes. A hypothesis named the forming of cavities and double electric layer is proposed. According to the hypothesis, ions migration within membranes form the ion-cavities inside membranes and the double electric layer beside membranes. It is the double electric layer that let counter-ions enter membranes from diluted water, and the ion-cavities that let counter-ions pass through the membranes and enter concentrated water. The hypothesis of the forming of cavities and double electric layers can explain the permselectivity of membranes on different occasions such as the electric field or non-electric field.