The equilibrium isotherms for adsorption of phosphate ions using an OH-type, strongly basic ion exchanger, DIAION SA10A, were studied experimentally and the results appeared technically feasible. Four different types of phosphate ions, H-3, PO4, H2PO4-, HPO42-, and PO43- were used. In all the systems considered, the experimental adsorption isotherms showed a high amount of phosphate adsorption. Moreover, differences in the adsorption behaviors were observed. These differences were highly influenced by the pH of the equilibrated solutions. The ion-exchange reaction models were then proposed. By the mass action law, the theoretical equations for the adsorption isotherms were derived and the unknown experimental equilibrium constants were determined. These equilibrium models were able to correlate the experimental data well and explained the peculiarities in the obtained experimental adsorption isotherms.