The electron transfer reaction between [Co(NH3)(5)(N-cyanopiperidine)](3+), pentaammine(N-cyanopiperidine)-cobalt(III), and hexacyanoferrate(II) has been studied in aqueous solutions, Typical saturation behavior was observed at high hexacyanoferrate(II) concentrations, which allows the precursor complex formation equilibrium constant, K-IP, and the true electron transfer rate constant, k(et), to be obtained, Variations of k(et) in the presence of various background electrolytes (sodium sulfate and sodium chloride) and surfactants (sodium dodecyl sulfate, SDS, dodecyltricosaethylene glycol ether, Brij(35) and octylphenol(ethylene oxide)(9.5) ether, Triton X-100) have been investigated at 298.2 K. Rationalization of the experimental data was assisted by the use of the Marcus theory on electron transfer, The conclusion is that micellar effects on the electron transfer rate constant can be explained by considering the micelles as a special background electrolyte with a high charge and a strong power of hydration.