Abstract Reaction of a methyltriphenylphosphonium (TPP) ion with a 2-mercaptobenzothiazole (MBT) monolayer on polycrystalline gold under electrochemical conditions leads to the chemical modification of the monolayer. Surface-enhanced Raman spectroscopy (SERS) has been used to study the molecular nature of the transformation. MBT, which adsorbs in the thione form on Au, gets transformed to the thiolate form upon reaction. Thermal stability of the modified monolayer is substantially lower than the corresponding pure monolayers and complete desorption of the modified monolayer occurs below 473 K. Time, potential, and solvent dependence of the reaction has been investigated. Electron transfer from the monolayer to the approaching ion in solution is suggested as the cause of the reaction. XPS investigation shows the change in the valence states of the species concerned. While the parent MBT gets oxidized, the TPP moiety gets reduced. The results suggest that modified monolayers can be made by simple electrochemical procedures and the processes are similar to the corresponding gas-phase events (1994, R. G. Cooks, T. Ast, T. Pradeep, and V. H. Wysocki, Acc. Chem. Res. 27, 316).