Physisorbed thiolates are found to be precursors in the oxidative chemisorption of hexadecanethiolates. In alkaline electrolyte solution, the physisorbed species undergo a slow, nonelectrochemical, reaction that shifts their oxidative adsorption to more positive potentials. This reaction, which makes the formation of a self-assembled monolayer more difficult, was monitored using second harmonic generation (SHG) spectroscopy. The SHG signal is found to be constant for the first 3 min that the thiolates are physisorbed. After this induction period, there is a slow decrease of the SHG signal that lasts 15 min. The magnitude of the SHG decrease is a function of the applied potential. A larger decrease is observed when the potential is close to the hydrogen evolution region. These results suggest that the reaction is a protonation of the physisorbed thiolates via a reaction with adsorbed hydrogen atoms. We also compare the electrodeposition of alkanethiolates with the adsorption of alkanethiols in absence of an applied electric field.