The binuclear complex formation between Fe(CN)(5)H2O3- and Co(en)(2)(2-pzCO(2))(2+) (en = ethylenediammine, pzCO(2) = pyrazinecarboxylate) has been studied in several aqueous electrolyte solutions and in sodium dodecyl sulfate, SDS, micellar systems in the presence and absence of various salts at 298.2 K. In dilute salt aqueous solutions the main factor operating on reactivity is the influence of the electrolyte concentration on the approaching process between the two ionic reactants of opposite sign, whereas in concentrated salt solutions the direct interaction cation-iron(II) complex seems to be the determining factor controlling changes in the reaction rate upon changing salt concentration In the case of the SDS micellar systems, kinetic micellar effects have been rationalized by considering the influence of interfacial electrical potential variations, with changing surfactant (or salt) concentration, on the process studied.