The relative quantum efficiencies for loss of cis-Rh(phen)(2)Cl-2(+) (when phen = 1,10-phenanthroline) in the presence of dA (deoxyadenosine), dG (deoxyguanosine), and uric acid have been measured in phosphate buffer. The relative reactivity was uric acid > dG > dA with the quantum efficiency in the presence of uric acid (under argon) approximately unity and that in the presence of dA essentially equal to that for cis-Rh(phenCl2+ alone. The product distribution was found to be oxygen dependent. cis-Rh(phenCl2+ aquation products were primarily formed in the presence of oxygen while adduct formation was the primary chemistry in the absence of oxygen. Reductive quenching mechanisms are invoked to account for the increased reactivity in the presence of dG (one electron) and uric acid (two electron). Such a mechanism provides a rationale for the previously observed site of covalent binding of the metal to dG.