The reversible "catch-and-release" of small molecules from the surface of monolayer-protected gold nanoparticles is described. The valency of the system (i.e., the number of molecules bound to the surface) can be controlled through the addition and removal of metal ions from the monolayer. Both the change in valency and the release rate of the molecules are strongly pH-dependent. The release rate can be regulated by altering the ratio of metal ions in the monolayer.