Bipolar membranes which consist of two layers having fixed positive and negative charge in series are theoretically examined for their ion separation ability in reverse osmosis on the basis of model calculation. The extended Nernst-Planck equation including the contribution of volume flux to salt permeation is used as the basic equation to describe the transport through bipolar membranes in reverse osmosis, to ether with electroneutrality, Donnan equilibrium and reverse osmosis conditions. By model calculation of single-electrolyte solutions, monopolar membranes show high rejection for electrolytes having divalent co-ions; that is, a negatively charged membrane rejects sodium sulfate much more than other electrolytes such as sodium chloride and magnesium chloride, while a positively charged membrane rejects magnesium chloride more than sodium sulfate. On the other hand, bipolar membranes reject electrolytes having divalent ions for both cation and anions more than mono-monovalent electrolytes; both sodium sulfate and magnesium chloride are rejected more than mono-monovalent electrolytes such as sodium chloride. Calculation of ion rejection in mixed electrolytes reveals that a bipolar membrane effectively separates ions according to their valences; divalent ions are rejected more than monovalent ones in mixtures. The charge structure of bipolar membranes, such as charge density ratio and thickness ratio of the two layers, is also discussed in terms of ion separation by bipolar reverse osmosis membranes.