The ligand substitution reaction [Fe(CN)(5)(4-(t)bupy)](3-) + pyrazine (4-(t)bupy=4-tert-butylpyridine) was studied in aqueous concentrated electrolyte solutions at 298 K. Plots of ln(k/k(w)) against (gamma-gamma (w)), where the subscript w refers to pure water and gamma is the surface tension of the appropriate salt solution, gave a common straight line for all the electrolytes studied and permits to estimate the activation volume.