Self-assembled monolayers of alkanethiols (CnH2n+1SH; n = 3, 8, 12, 18, and 22) adsorbed on gold(111) are investigated with (atomically resolved) scanning tunneling microscopy (STM) and wetting measurements. The characteristic depressions observed in these monolayers with STM are proven to be holes in the underlying top gold surface layer rather than defects in the thiol monolayer itself. The holes originate from an etching process of the gold during adsorption of the thiol molecules : a correlation is obtained between the number of holes observed with STM and the amount of gold measured with atomic absorption spectroscopy in the thiol solution after adsorption. The erosion process is found to vanish as soon as complete self-assembly is observed in STM and wetting. For a dodecanethiol monolayer on gold adsorbed from a diluted methanoic thiol solution, self-assembly is observed within 10-min adsorption time. The average amount of gold in the thiol solution after 10 min corresponds to dissolution of 2% of a monolayer Au(111). The erosion strongly increases when the dodecanethiol adsorbs from undiluted thiol. The amount of holes also increases with decreasing thiol chain length as a result of a lower degree of self-assembly. The surface gold atoms underneath the thiol layer are highly mobile, which is manifest in STM tip-induced reorganization of the thiol layer and in penetration of evaporated gold through the thiol layer. This mobility is believed to be crucial in the etching process. Due to the mobility of thiol molecules during the adsorption process prior to acquiring a complete self-assembled structure, gold dissolves, probably in the form of a gold thiolate complex.