The ligand substitution reaction between Fe(CN)(5)H2O3- and beta-Co(trien)(2)(pzCO(2))(2+) (trien = triethylenetetraammine; pzCO(2) = pyrazinecarboxylate) was studied in aqueous salt and sodium dodecyl sulfate micellar solutions. In homogeneus aqueous solutions, an increase in the concentration of the background electrolytes resulted in a decrease in the observed rate constant. This was explained by considering the influence of the salts on the activity coefficients of the participants in the reaction using a Guggenheim-Guntleberg-type equation. In sodium dodecyl sulfate micellar solutions, the effect of the variation of the surfactant concentration operated mainly through its influence on the approaching process between the two oppositely charged reactants. When the surfactant concentration is high enough to ensure the total incorporation of the Co(III) complex molecules into the micellar pseudophase, the decrease in the electrical component of the free energy of activation (controlled by changes in the interfacial electrical potential, Delta Psi) is responsible for the increase in the observed rate constant when [SDS] increases; The addition of salts and alcohols to the sodium dodecyl sulfate micellar reaction media provokes an increase in the observed rate constant, this increase being greater the higher the [additive]. This can be explained considering that the presence of these additives results in a diminution of the electric component of the free energy of activation due to a decrease in \Delta psi\ of the negatively charged micelles.