Equations were determined for the calculation of the stoichiometric (molality scale) dissociation constant Km of benzoic acid in dilute aqueous NaCl and KCl solutions at 25°C from the thermodynamic dissociation constant Ka of this acid and from the ionic strength Im of the solution. The salt alone determines mostly the ionic strength of the solutions considered in this study and the equations for Km were based on the single-ion activity coefficient equations of the Hückel type. The existing literature data obtained by conductance measurements and by electromotive force (EMF) measurements on Harned cells were first used to revise the thermodynamic value of the dissociation constant of benzoic acid. A value of Ka = (6.326 ± 0.005) × 10−5 was obtained from the most precise conductivity set [Brockman and Kilpatrick] and this value is supported within their precisions by the less precise conductivity set of Dippy and Williams and by the EMF data set measured by Jones and Parton with quinhydrone electrodes. The new data measured by potentiometric titrations in a glass electrode cell were then used for the estimation of the parameters of the Hückel equations of benzoate ions. The resulting parameters were also tested with the existing literature data measured by cells with and without a liquid junction. The Hückel parameters suggested here are close to those determined previously for anions resulting from aromatic and aliphatic carboxylic acids. By means of the calculation method based on the Hückel equations, Km can be obtained almost within experimental error at least up to Im of about 0.5 mol-kg−1 for benzoic acid in NaCl and KCl solutions.