Quartz crystal microbalance and scanning tunneling microscopy experiments have been used to demonstrate that self-assembled monolayers of thiophene on gold can be quantitatively replaced by absorption of methanethiol from the gas phase. Adsorption of thiophene on the gold surface results in the relaxation of the clean gold surface reconstruction and the formation of vacancy islands or "pits" in the gold. Subsequent annealing of the monolayer can result in the complete removal of the vacancy islands. Methanethiol displacement of thiophene monolayers that have been previously annealed to remove vacancy island "pits" does not generate additional "pits" in the gold surface. Thus, ordered methanethiol monolayers which are free from vacancy island "pits" in the underlying gold surface can be formed by displacement of annealed thiophene monolayers. These results provide strong additional evidence that the formation of vacancy islands in the gold during the formation of alkanethiol SAMs is directly associated with relaxation of the clean surface reconstruction, rather than conventional chemical etching of the surface by alkanethiol.