We have used applied electrical potentials, in combination with self-assembled monolayers (SAMs) of alkanethiolates supported on gold films, to control the wettability of a surface over wide ranges. A surface can be transformed from nonwetting to wetting, or the reverse, with time constants of seconds. The method is based on a competition between reductive electrochemical desorption of a hydrophobic SAM and its re-formation from alkanethiol in solution. Self-assembled monolayers formed from either CH3(CH2)(15)-SH or CH3(CH2)(2)SH were hydrophobic (80 degrees < theta < 110 degrees, theta = contact angle) toward aqueous solutions of electrolyte at neutral potentials but became hydrophilic (theta similar to 10 degrees) at reducing potentials (E < -1.3 V vs Ag wire) : Contact angles of aqueous solutions containing CH3(CH2)(2)SH returned to their initial values (theta similar to 80 degrees) after the reducing potentials were removed. Because the change in wettability was dependent on the structure of the organic molecule in the monolayer, it was possible to prepare patterned SAMs in which certain regions were transformed from hydrophobic to hydrophilic by changing potential, while other regions were inert.