Catalytic guanine oxidation using Ru(bpY)(3)(2+) as a mediator (bpy = 2,2'-bipyridine) has been investigated electrochemically in viscous solutions containing high concentrations of poly(ethylene glycol) (PEG, average m.w. 1000). The electrocatalytic cyclic voltammograms contain prepeaks, which typically are not observed for aqueous solutions-of DNA without PEG-1000. The prepeak is not due to adsorption of the oxidized mediator because prepeaks were not observed when cyclic voltammograms were collected for solutions without nucleic acid or with nucleic acid that did not contain guanine. At fast scan rates, the prepeak disappears completely, suggesting that the prepeak is due to substrate depletion at the electrode surface as a result of fast catalysis. In the presence of the high concentrations of PEG-1000 used in this study, radiolabeling of small DNA fragments showed that nucleic acids containing substrate guanine adsorb to the electrode surface. An explanation for the substrate depletion may be that the adsorbed DNA has a higher rate constant for guanine oxidation than DNA in solution, as reported previously for a system in which all of the DNA was adsorbed [Armistead, P. M.; Thorp, H. H. Anal. Chem. 2001, 73, 558-564]. Digital simulation using two noninterconvertible DNA populations, one fraction with a large rate constant for guanine oxidation and the other fraction with a rate constant typical of guanine oxidation in solution, generated prepeaks in the simulated cyclic voltammograms. We conclude that increasing the viscosity of solutions containing DNA and Ru(bpy)32+ by adding PEG-1000 promotes DNA adsorption and that this combination of adsorption, viscosity, and fast electrocatalytic oxidation of adsorbed DNA produces prepeaks in cyclic voltammograms.