The electrocatalytic reduction of ferricyanide mediated by methylene blue (MB) at a DNA-modified electrode was investigated by linear-sweep voltammetry at a rotating gold-disk electrode. Electrodes were modified with thiol-terminated double-stranded oligonucleotides (sequence: 5?-SH-AGTACAGTCATCCGC-3?) to form densely packed DNA films that blocked the direct electrochemical reduction of mM solutions of ferricyanide at overpotentials greater than 0.5 V. Addition of mM concentrations MB to these solutions, however, resulted in the rapid appearance of catalytic currents as long as the potential was held negative of the formal potential of MB within the DNA film (-0.30 V vs. SCE). MB binds to the DNA-modified surfaces under these conditions, and the adsorption kinetics of MB were determined by a coulometric assay. These data fit a simple Langmuir model with <i style="mso-bidi-font-style:normal">kon = 2(1) x 10<sup>4</sup> M<sup>-1</sup>s<sup>-1</sup> and <i style="mso-bidi-font-style:normal">koff = 0.02(2) s<sup>-1</sup>. Interestingly, linear-sweep RDE voltammograms recorded in the presence of MB revealed ?peaked?-shaped i-V traces, which took several seconds to reach their steady-state values. Moreover, the steady-state current density for reduction of Fe(CN)6<sup>3-</sup> was found to depend on the bulk concentration of MB in solution. A model is proposed to account for these data, in which the catalytic reduction of Fe(CN)6<sup>3-</sup> is limited ultimately by the kinetics of MB crossing into and out of the film, and not DNA-mediated charge transfer through the monolayer.