Abstract Electroactive monolayers can be formed in which the redox centers are held at a fixed and controllable distance from the electrode. Thiols with attached redox centers (HS(CH 2) n CONHCH 2pyRu(NH 3) 5 2+/3+, n = 10, 11, 15) readily adsorb from acetonitrile solutions onto gold electrodes to form electroactive monolayers. Mixed monolayers are synthesized by co-adsorbing the electroactive thiols with diluent alkanethiols (HS(CH 2) n CH 3, n = 11, 15) and ω-mercaptoalkanecarboxylic acids (HS(CH 2) n COOH, n = 10, 11, 15). Cyclic voltammetry of the coated electrodes shows a stable redox wave near 0 V vs. SCE in pH 4 aqueous electrolyte. Coverage of the redox centers in mixed monolayers is controllable by adjusting the mole ratio of the electroactive thiol to the diluent thiol in the deposition solution. At sufficiently slow scan rates, cyclic voltammograms of the electroactive monolayers are nearly ideal (Δ E p = 0 mV and Δ E fwhm = 90 to 100 mV) for both diluent thiols and for all coverages of the redox centers. The evidence indicates that the monolayers are well ordered and that the redox centers reside at the interface between the aqueous phase and the hydrocarbon phase. These monolayers are suitable for the study of electron tunneling across the monolayer.