The electrochemical underpotential deposition (UPD) of metal monolayers of a series of metals at Au electrodes modified with self-assembled monolayers (SAMs) of (3-mercaptopropyl)trimethoxysilane (3MPT) is presented. Metal monolayers are deposited from aqueous solutions containing the precursor ions of interest, including Tl⁺, Pb⁺², Cu⁺² and Cd⁺². Cyclic voltammetric characterization of underpotential monolayer formation of these metals permits elucidation of the thermodynamic and kinetic effects involved in metal deposition beneath the cross-linked 3MPT SAM film. Results substantiate a mechanism previously proposed from this laboratory for deposition of metal monolayers beneath 3MPT SAMs on Ag. The varying Stokes radii of the investigated metal ions provide insight into the structure of the hydrolyzed and condensed siloxane rings of 3MPT SAMs on Au. STM images of 3MPT-modified Au surfaces corroborate the conclusions of the electrochemical studies regarding the structure of the siloxane terminus of these SAMs.