The inhibition by an adsorbed nonionic surfactant (sorbitan monostearate) of electron transfer across an aqueous/organic interface, between ferro-/ferricyanide and dimethyl (DiMFc) or decamethyl (DcMFc) ferrocene, is related to the fraction of the surface covered. However, the apparent surface geometry depends on the direction of electron transfer: the blocked interface behaves as an array of isolated microelectrodes for the oxidation of DiMFc or DcMFc and as a uniformly accessible surface for the reduction of DiMFc(-). Interfacial capacitance and interfacial tension measurements gave the surface coverage of the surfactant, and scanning electrochemical microscopy approach curves yielded the overall reaction rate constant fur ferrocene oxidation. Four-electrode voltammetry revealed the asymmetry of the interface geometry induced by surfactant adsorption. It is concluded that sorbitan monostearate forms patches on the interface, which the uncharged ferrocene derivative cannot penetrate. The packing density of surfactant molecules in the surface patches increased with increasing surface coverage. From the limiting current values fur DiMFc oxidation obtained from cyclic voltammograms for near-complete coverage of surfactant, the diameter of the holes in the surfactant layer is calculated to be approximately 200 nm. In contrast, it semis that the oxidized, charged ferricenium can induce a local rearrangement of the adsorbed layer and hence diffuse freely through the layer.