The electrochemical quartz crystal microbalance (EQCM) has been applied to the silver/ethyl xanthate system, and the derived mass change was found to track the development of silver xanthate but not underpotential xanthate deposition. The resonant frequency recorded on sweeps through the latter potential region moved in the opposite direction to that expected from mass changes. This is explained by the transition from hydrophilicity to hydrophobicity of the surface resulting from xanthate chemisorption. Analogous behavior was observed during sulfur deposition on platinum and gold; these systems display similar potential dependence of wettabilty. For sulfur deposition on silver and for oxygen adsorption on platinum, the EQCM measured mass changes because each surface remains hydrophilic during the potential excursions imposed. Similarly, oxygen adsorption was measured by the EQCM on a fresh gold surface in acid media, but not after the surface was restructured on potential cycling, nor for gold in basic media. This is explained by the minimum interaction between gold and water at the interface. (C) 2001 The Electrochemical Society. All rights reserved.