The kinetics of the electron-transfer reaction between pentaaminepyrazine ruthenium(II), [Ru(NH3)(5)pZ](2+), and trisoxalatecobaltate(III), [CO(C2O4)(3)](3-), was studied in sodium bis(2-ethylhexyl)sulfosuceinate-oil-water microemulsions and in micellar solutions of hexadecyltrimethylammonium chloride. The results can be interpreted by taking as a starting point the pseudophase model. However, the model must be modified according to the characteristics of the reaction media. In this way, two different equations result for this model in the two reaction media studied. The influence of the surface potential on the parameters (equilibrium and rate constants) of the model is shown.