Published experimental thermodynamic data at 298.15 K for aqueous mixtures of H2Ph, KHPh, and KCl, of KHPh, K2Ph, and KCl, and of KHPh and KCl, where H2Ph means phthalic acid, KHPh potassium hydrogen phthalate, and K2Ph dipotassium phthalate, were used to test the methods for calculation of the pH values of phthalate buffer solutions. Equations for ionic activity coefficients are used in these methods. It is shown that all data used, up to an ionic strength of about 0.5 mol kg(-1), can be predicted almost within experimental error by two methods. In one of these methods, equations of the modified Guggenheim type are used for ionic activity coefficients, and in the other equations of the Fitter type are used. Several sets of phthalate buffer solutions are suggested in the pH range 3.8-5.3, e.g., for calibration solutions for glass electrode cells or for constant-pH reaction media where the pH is known. In the recommended sets, the pH values of the buffer solutions have been calculated by the Guggenheim method, and the pH predictions of the Fitter method agree with those in most cases within 0.01 at least up to ionic strengths of about 0.15 mol kg(-1). The recommended pH = 4.005 of the standard reference solution (0.05 mol kg(-1) KHPh solution) agrees closely with the value suggested in the present study (4.003) for this solution.