Equations were developed for the calculation of the second stoichiometric ( molality scale) dissociation constant ( K-m2) of phthalic acid in buffer solutions containing potassium dihydrogen phthalate, hydrogen phthalate, and 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 . kg(-1), and they are based on the single-ion activity coefficient equations of the Huckel type. The parameters of phthalate ions for these equations and the thermodynamic values of the second dissociation constant of this acid at various temperatures were determined from the Harned cell data measured by Hamer and Acree ( J. Res. Natl. Bur. Stand. 1945, 35, 381- 416). In these parameter estimations, the stoichiometric values of the first dissociation constant of phthalic acid were needed, and these values were obtained from the Harned cell results of Hamer et al. ( J. Res. Natl. Bur. Stand. 1945, 35, 539-564) with some reasonable extra approximations. All calculations from the data of Hamer and Acree were completely revised, and all parameters estimated depend in a simple way on the temperature. The interaction parameters between hydrogen and chloride ions were taken from results of a previous HCl paper ( Partanen, J. I.; Covington, A. K. J. Solution Chem. 2002, 31, 197-210). The parameters resulting from interactions between hydrogen and potassium ions and from interactions between potassium and chloride ions were taken from a recent study ( Partanen, J. I.; Covington, A. K. J. Chem. Eng. Data 2005, 50, 497- 507), where these parameter values were determined from Harned cell data of Harned and Hamer ( J. Am. Chem. Soc. 1933, 55, 2194-2206) in HCl + KCl solutions. The resulting simple equations for calculation of K-m2 for phthalic acid were tested with the data from the parameter estimation and apply well to these data. In the second part of this study, these equations will be additionally tested with good results on all reliable data found in the literature from the first and second dissociation reactions of phthalic acid in KCl solutions.