The thermodynamics of the complex formation reactions of copper(II) and iron(III) with benzoate ions have been determined by measuring the formation constants at different temperatures. The benzoate complexes of fairly hard acceptors, like Fe(III), are characterized by highly positive entropy changes. All measurements were carried out potentiometrically at 25, 35 and 40-degrees-C and in an ionic medium of 1 M, with sodium perchlorate as the background electrolyte.