A monolayer constituted by chemisorbed 6-thioguanine and physisorbed guanine molecules was formed on a hanging mercury drop electrode from deoxygenated solutions of 6-thioguanine (6TG) and guanine (G). The mixed monolayer was characterized by chronoamperometry, cyclic voltammetry, and phase-sensitive ac voltammetry under in situ conditions, and the experimental data revealed a compact packing of molecules in the film. Ex situ measurements in the presence of dissolved oxygen proved that the mixed monolayer is stable in solutions lacking 6TG and G. (C) 2004 Elsevier Inc. All rights reserved.