Equations were determined for the calculation of stoichiometric (molality-scale) dissociation constants, K-m, of monoprotic aliphatic carboxylic acids in buffer solutions containing formic acid, acetic acid, or some other weak acids of this kind and potassium chloride from the thermodynamic dissociation constants, Ka, of the acids and the molalities of the components in the solutions. These equations apply for formic acid to temperatures from (0 to 60) degrees C and were based on the single-ion activity-coefficient equations of the Huckel type. The parameters of formic acid species for these equations and the thermodynamic dissociation constants of this acid at various temperatures were determined from the Harned cell data of Harried and Embree (1934). All calculations from these data were revised, and the final parameter values obtained are independent of the temperature. The interaction parameters between hydrogen and chloride ions were taken from the results of a previous HCl paper (2002). The parameters that resulted from interactions between hydrogen and potassium ions and between potassium and chloride ions were determined from the Harned cell data of Harned and Hamer (1933) in HCl and KCl solutions. The resulting simple equations for formic acid were tested, in addition to the revised data used in the estimation, with the existing literature data sets measured on galvanic cells with and without a liquid junction at various temperatures. The new equations predict well the data used in the tests. The new equations were used to predict pH and P(m(H)) values of the formate buffers that contain equal molalities of (0.01 or 0.1) mol kg(-1) of formic acid and potassium formate at temperatures from (0 to 60) degrees C. The p(m(H)) values, calculated by these equations, were also tabulated for buffer solutions containing KCl as the major component and formate species as minor components for the calibration of a glass electrode cell in such a way that it measures, directly after calibration, the molality of hydrogen ions. The Huckel equations were also applied to a re-evaluation of the activity parameters of acetate species from the literature data obtained from Harned cells for aqueous mixtures of acetic acid and KCl The resulting values agree quite well with those obtained here for formate species. It is also shown that the resulting parameter values apply well to species from acetic, formic, propionic (propanoic), butyric (butanoic), 2-methylpropanoic, pentanoic, 2,2-dimethylpropanoic, hexanoic, 4-methylpentanoic, and heptanoic acids from the quinhydrone electrode data of Larsson and Adell (1931) measured in aqueous KCl solutions at 18 degrees C.