The substitution reaction of bis-ethylenediammine(2-pyrazinecarboxylato)cobalt(III) for acetonitrile ligand in acetonitrile pentacyanoferrate(II) was studied in micellar solutions of sodium dodecyl sulfate. The results are interpreted considering the influence of the electric geld in the micellar interfacial region on the formation of the encounter complex between the reactants. The influence of the field on the reverse (dissociation) process is also discussed.