Previously (in Part 1, J. Chem. Eng. Data 2006, 51, 777-784), equations were determined for the calculation of the second stoichiometric (molality scale) dissociation constant (K-m2) of phthalic acid in buffer solutions containing potassium hydrogen phthalate (KHPh), dipotassium phthalate, and potassium chloride from the determined thermodynamic values of this dissociation constant (K-a2) and the molalities of the components in the solutions. These equations apply at temperatures from (0 to 60) degrees C up to ionic strengths of about 0.5 mol center dot kg(-1), and they were based on the single-ion activity coefficient equations of the Huckel type. The parameters of phthalate species for these equations and the second thermodynamic dissociation constant of this acid at various temperatures were determined from Harned cell data of Hamer and Acree (J. Res. Natl. Bur. Stand. 1945, 35, 381-416), and the resulting parameter values were also tested with these data. In the present study, the corresponding equations were determined from the Harned cell data measured by Hamer et al. (J. Res. Natl. Bur. Stand. 1945, 35, 539564) in phthalic acid, KHPh, and KCl solutions for the first dissociation constant of this acid at various temperatures, and the resulting equations were also tested. Additionally, the new models for calculation of K-m1 (determined in this part) and Km2 (determined in Part 1) were tested with the plentiful Harned cell data existing in the literature for 0.05 mol center dot kg(-1) KHPh solutions containing KCl at temperatures from (0 to 60) degrees C. The models usually apply well to these data. The new activity coefficient equations were used to evaluate pH values of the phthalate buffer solution (i.e., of the 0.05 mol, kg(-1) KHPh solution) for comparison with the pH values recommended by IUPAC (Pure Appl. Chem. 1985, 57, 531-542; 2002, 74, 2169-2200) at temperatures from (0 to 60) degrees C. Values of p(m(H)), which give the molality of protons directly, calculated from these equations are also tabulated for these buffers as well as for buffer solutions with KCl and KHPh as the major component and minor component, respectively.