Photoinduced electron transfer reactions between photoexcited Ru(phen)(2)dppz(2+) (phen = 1,10-phenanthroline, dppz = dipyridophenazine) and accepters Rh(phi)(2)bpy(3+) and Rh(phen)(2)phi(3+) (phi = 9,10-phenanthrenequinone diimine, bpy = 2,2’-bipyridine) are compared in micelles and DNA. Both microheterogeneous environments contain a negatively charged surface and hydrophobic interior and the cationic complexes associate strongly with each. However, reactions between molecules bound to DNA or to micelles show striking differences which can be correlated with the unique character of the highly ordered, pi-stacked basepairs in DNA compared to the disordered, aliphatic chains in the micelles. In DNA, Rh(phi)(2)bpy(3+) quenches *Ru(phen)(2)dppz(2+) on a fast time scale (unimolecular rate constant greater than or equal to 10(8) s(-1)), whereas no detectable quenching of *Ru(II) emission by Rh(phen)(2)phi(3+) is observed. In contrast, both complexes quench equally well in SDS micelles. Although static quenching on the nanosecond time scale is observed for Rh(phi)(2)bpy(3+) in DNA, reactions in SDS occur dynamically by intramicellar diffusion, with a bimolecular rate constant of 1.1 x 10(8) M(-1) s(-1) for Rh(phi)(2)bpy(3+) and 1.2 x 10(8) M(-1) s(-1) for Rh(phen)(2)phi(3+). Reactions on DNA are also shown to be DNA-mediated in that no solvent-isotope effects are observed in the quenching. In addition, there is enantioselectivity seen in reactions on the right-handed DNA helix but not in the achiral micelle, indicating that quenching is sensitive to the geometry of intercalation. Efficient electron transfer quenching in DNA compared to SDS micelles therefore provides evidence against the cooperative association of molecules on DNA and for the importance of intercalative stacking of the donor and acceptor for fast electron transfer through the DNA pi-stack.