A montmorillonite membrane with a thickness of 0.5 mm was prepared, and reverse osmosis experiments, were conducted for various inorganic and organic solutes of different sizes under an operating pressure of 3 MPa. The mean pore size and pore size distribution were determined by a log-normal probability plot between the solute separation and the Einstein-Stokes radius of the solute molecules for which reverse osmosis and ultrafiltration performance was investigated. The relationships between the pore size and the pore size distribution and the structure of montmorillonite are discussed. A comparison is also made between the pore size and the thickness of the electrical double layer.