We calculated the characteristics of a phosphoric cation exchanger and studied an accurately computable method for ion exchange capacity for a type of potentiometric titration curve. The ion exchanger was prepared by phosphorylation of a styrene-divinylbenzene copolymer. The ion exchange capacity was 5.7 meq/g. The experimental pK values versus chi in a phosphoric cation exchanger can explain a linear equation. The Delta pK values were obtained from the slope of a linear equation. The Delta pK values were the differences of pK values between the apparent equilibrium constant at complete and zeroth neutralization of the ion exchanger. The experimental pK values at chi = 0.5 (chi :degree of neutralization of ion exchanger) showed good agreement with the theoretical data. When it was titrated with NaOH and Ba(OH)(2) solutions, a good agreement between experimental and theoretical pK values for various chi was found in all potentiometric titration curves. The potentiometric titration curve near the inflection point in the case of divalent ions was changed more sharply than that for monovalent ions. The plot of partial derivative pH/partial derivativeg versus g (number of moles of alkali to 1 g of ion exchanger) was fitted to the Lorenzian distribution, from which ion exchange capacity was accurately evaluated.