The place-exchange reaction of thiol-containing peptides in a cationic monolayer on gold nanoparticles occurs very rapidly at low micromolar concentrations in water with excellent control over the degree of substitution. The driving force for this process is the binding of anionic peptides to a cationic monolayer surface which causes a strong increase in the local concentration of thiols. The place-exchange reaction can be triggered by light using a photolabile protecting group on the thiol moiety.