Underpotential deposition (UPD) of Ag on Au(111)/mica electrodes coated with a self-assembled monolayer (SAM) of propanethiol or octanethiol and reductive desorption of the SAM after conducting UPD have been studied using voltammetry, XPS measurement, and scanning tunneling microscopy (STM). The reductive desorption potential of the SAM was changed by UPD of Ag from a characteristic value obtained at Au to that at Ag, indicating that the UPD of Ag took place through the SAM layer in such a way as to intervene between the SAM and the Au electrode. No significant loss of thiol molecules occurred during the Ag deposition. The rate of UPD through the SAM of propanethiol was so fast as to be completed within 10 s, while that for the SAM of octanethiol took ca. 50 min or more to build up the SAM/Ag/Au structure. Voltammetric results indicated that the UPD of Ag proceeded initially at molecular defects in the SAM of octanethiol and that the resulting Ag islands grew laterally to limiting coverage. Ex situ STM observations showed clearly the presence of such Ag islands on terraces of the Au(111) electrode surface.