Polyacrylic acid membrane which was sandwiched by cellulose acetate membranes was prepared and tested in reverse osmosis. Salt rejection was found to be a function of pressure, feed concentration, and ionic valence. It is qualitatively in agreement with a Donnan exclusion theory. Rejections of monovalent counter-ion salts increased strongly with decreasing concentration, while those of divalent counter-ion salts are not so sharp against decreasing concentration. Salts of divalent co-ion are more highly rejected than those of monovalent co-ion. It was found that salt rejections were connected with a change of pore volume or water content in membrane caused by pressure, feed concentration, and valence of ions.