Industrial additives, like sodium benzoate and potassium benzoate, are used in low concentrations in aqueous solutions. However, in general, the ionic strength of the aqueous media is high. The Pitzer model allows the determination of activity coefficients in aqueous media. In this work, Pitzer parameters for sodium benzoate were recalculated (beta((0)) = 0.162 mol(-1) kg and beta((1)) = 0.38 mol(-1) kg) and Pitzer parameters for potassium benzoate were determined (beta((0)) = 0.157 mol(-1) kg and beta((1)) = 0.27 mol(-1) kg) from cell potential difference (cpd) measurements on galvanic cells without liquid junction. From these values, activity coefficients of potassium benzoate in water and in aqueous potassium chloride solutions were calculated by the Pitzer model. The activity coefficients were also calculated by modified Guggenheim equations with parameters obtained in this work and the results of the two models were compared. (C) 2003 Elsevier B.V. All rights reserved.