Aqueous solutions of 0.1 M NaClO4 (pH 1.5, 10 mM KH2PO4) show potential-dependent wetting on self-assembled monolayers (SAMs) formed from 15-(ferrocenylcarbonyl)pentadecanethiol (FcCO(CH2)15-SH; Fc = [eta5-C5H5]Fe[eta5-C5H5]) adsorbed on Au surfaces. Contact angles (theta) decreased from 71-degrees to 43-degrees (DELTA cos theta = -0.40) when the electrical potential of the SAM was increased from 0.3 to 0.5 V (vs a Ag wire reference electrode) and then increased from 43-degrees to 58-degrees when the potential of the SAM was returned to 0.2 V. Repeated cycling between these values of the potential leads to a progressively decreasing response, as the Fc groups were destroyed by side reactions. Contact angles of aqueous solutions on SAMs formed from CH3(CH2)17SH decrease by only 2-degrees (from approximately 115-degrees to 113-degrees, DELTA cos theta = 0.05) over the same range of potentials (Sontag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1992, 8, 2560-2566). The contrast between the wettability of SAMs terminated with Fc and CH3 groups suggests that potential-dependent wetting of the former is caused primarily by the electrochemical oxidation of the electrically neutral, surface-confined Fc to the more polar and plausibly more wettable Fc+ cation. Linear sweep cyclic voltammetric measurements support this hypothesis. Surfaces of gold patterned with SAMs formed from FcCO(CH2)15SH and CH3(CH2)15SH were used to construct a micrometer-scale "gate" that controls the flow of liquid down a surface.