The kinetics of the electron-transfer reaction between pentaamminepyridine ruthenium (II) perchlorate, [Ru(NH3)(5)py](ClO4)(2), and sodium peroxodisulfate, Na2S2O8, was studied in DNA and SDS micellar solutions. 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. Thus, the binding of the substrate (the ruthenium complex) to DNA is anticooperative in character, in such a way that the binding constant increases as the [DNA]/[Ru(NH3)(5)py](2+) ratio increases. On the other hand, the binding of the ruthenium complex to SDS micelles decreases as the [SDS]/[Ru(NH3)(5)py](2+) ratio increases as a consequence of the changes of the surface potential of the micelles.