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The kinetics of the substitution of Br~b y OHin the bromopentaamminecobalt (III) complex has been investigated spectrophotometrically in the presence of ten different supporting electrolytes, at concentrations up to 2.7 M. The influence of electrolytes having univalent anions can be represented by a classical expression, whereas no quantitative explanation has been found for the influence of SO,-2. The connection between the salt effect and the possible existence of a rapid pre-equilibrium is discussed. The classical salt effect in

The influence of supporting electrolytes on the equilibrium has been investigated with a view to comparing this influence with that exerted on the rate s of reactions of similar charge-types. Neither the influence on the equilibrium constant nor that on the rate constant of the back reaction was fonnd to be uniquely determined by the principle of ionic strength. These results, together with examples from the literature, show that there seems to be nothing special about environmental influence on the kin e tic s of reactions. It is suggested that what makes prediction difficult is the fact that in both cases activity coefficients appear in a combination which is different from that encountered in the mean activity coeffic tent, A spectrophotometric method is employed. The value of the equilibrium constant at zero ionic strength has been found equal to 4,4 x 10-2 ; the heat of reaction is 3.8 Kcal mol-I. The discrepancies between these values and those reported in the literature are discussed.