Equations were determined for the calculation of the stoichiometric (molality scale) dissociation constant, K-m, of lactic acid in aqueous salt solutions at 291.15 and 298.15 K from the thermodynamic dissociation constant, K-a, of this acid and from the ionic strength, I-m, of the solution. The salt alone determines mostly the ionic strength of the solutions considered in this study, and the equations for K-m were based on the single-ion activity coefficient equations of the Huckel type. New data measured by potentiometric titrations in a glass electrode cell at 298.15 K and the literature data obtained by Larsson and Adell with quinhydrone (qh) electrode cells at 291.15 K were used in the estimation of the parameters for the Huckel equations (HE) of lactate ions in NaCl and KCl solutions. The Harned cell data measured at 298.15 K by Nims and Smith were used to obtain the activity parameters for lactate ions in dilute LiCl, BaCl2 and SrCl2 solutions. The conductance data measured at 298.15 K by Martin and Tartar were used to determine the thermodynamic value of the dissociation constant of lactic acid. By means of the calculation method suggested in this study, Km can be obtained almost within experimental error at least up to I-m of about 0.5 mol kg(-1) for lactic acid in NaCl and KCl solutions at the two temperatures considered. The K-m values obtained by this method were compared to those obtained by the calculation methods presented recently in the literature for a general treatment of thermodynamics of weak acids in NaCl and KCl solutions.