Аннотация:A generalized Debye–Hückel theory is used to calculate coefficients of activity for water and ionsin solutions of alkali metal formates at 298 K, according to experimental values of their static permittivity. It is shown that the model qualitatively reproduces the nonmonotonic dependence of the coefficients of activity in sodium and potassium formate solutions. The effect hydration and ionic association have on their thermo-dynamic properties in a number of the studied systems is discussed.