The adsorption of dodecyl mercaptan (DM) monolayers onto plasma preoxidized gold was studied. The degree of preoxidation was determined quantitatively using electrochemical stripping analysis. It is shown that (i) alkanethiols form oriented monolayers on gold oxide surfaces. (ii) The contact angles and ellipsometric parameters of DM monolayers on gold vary systematically with the degree of preoxidation of the evaporated gold substrate. (iii) The monolayer effectively isolates the oxide from its surrounding, thus preventing chemical and electrochemical reduction of the inherently unstable gold oxide. The existence of a layer of encapsulated gold oxide under the organic monolayer is verified by X-ray photoelectron spectroscopy (XPS) and combined electrochemical-depth profiling measurements.