X-ray photoelectron spectroscopy (XPS), low energy electron diffraction, and cyclic voltammetry have been used to study the adsorption of iodine on the Ru(0001), air, and water exposure to both clean and iodine covered Ru(0001) surfaces and the stability of the iodine adlayer during Cu overpotential electrodeposition. A Ru(0001)-(root3 X root3)R30degrees-I structure was observed after I-2 vapor exposure of the Ru(0001) surface at room temperature. The Ru(0001)-(root3 X root3)R30degrees-I structure was found to be stable toward ambient air and water exposure. The I ad-layer passivates the Ru(0001) surface against significant hydroxide, chemisorbed oxygen, or oxide formation during exposure to air. Immersion of I-Ru(0001) results in greater hydroxide and chemisorbed oxygen formation than air exposure. A saturation coverage of I on a Ru(poly) electrode similarly passivated the Ru surface against oxidation upon exposure to water vapor over an electrochemical cell in an ultrahigh vacuum electrochemistry transfer system. Studies with combined electrochemical and XPS techniques show that iodine surface adlayer remained on top of the surface after cycles of overpotential electrodeposition/dissolution of copper on Ru(0001). These results indicate the potential bifunctionality of the iodine layer to both passivate the Ru surface in the microelectronic processing and to act as a surfactant for copper electrodeposition. (C) 2004 The Electrochemical Society.