In an attempt to develop materials to remove phosphorus that causes eutrophication or red tide, boehmite (aluminum oxide hydroxide) was surface-treated with sulfuric acid, nitric acid, hydrochloric acid, and sulfates, and the adsorption rates and adsorption isotherms of phosphate ions onto them were measured. The amount of phosphate ions adsorbed onto the boehmite, which was treated with sulfuric acid, was the greatest. This suggests that hydroxyl groups located in the boehmite are exchanged for sulfate groups by the sulfuric acid treatment. When boehmite was surface-treated with sulfuric acid of different concentrations, the amount of phosphate ions adsorbed increase with the increasing concentration of sulfuric acid. This result indicates that the adsorption of phosphate ions can be controlled by the amount of sulfate groups introduced in boehmite. The sulfate group of boehmite could also be introduced by an acidic sulfate solution treatment. When boehmite was treated with sulfuric acid, its capacity to adsorb a low concentration of phosphate ions from sea water increased to approximately 3 times that of the untreated boehmite. This suggests that the boehmite after the treatment of sulfuric acid or an acidic sulfate solution could be utilized for removing phosphate ions from sea water.