The self-assembly of alkanethiols on underpotentially deposited Ag(1x1)-Au(111) has been studied by using scanning tunneling microscopy, Auger electron spectroscopy, and electrochemical techniques. Even for short adsorbed alkanethiolates, the surface structure consists of an incommensurate hexagonal lattice with nearest-neighbor distances; approximate to 0.48 nm that are usually found for long alkanethiolates adsorbed on the Ag(111) surface. Surprisingly, the stability of the self-assembled alkanethiolate monolayers against reductive electrodesorption is increased in similar to0.10 and 0.40 V with respect to those observed on Ag(111) and Au(111), respectively. Density-functional theory calculations for methanethiolate desorption from a model cluster indicate that the enhanced stability arises from a balance between the energy to introduce an electron into the alkanethiolate-Ag(1x1)-Au(111) system and the alkanethiolate desorption energy that is strongly modified by the An substrate.