In summary, this study sheds new light on one of the essential questions concerning the nature of thiol-based ligand exchange on colloidal metal nanoparticles, which has been difficult to address before the use of the isotopically labeled SERS spectroscopy approach proposed here. Our results provide important insight into the exchange dynamics of surfacebound molecules and draw attention to the nanoscale peculiarities of surface-functionalized colloidal metal substrates. The hidden phase transition, revealed here, may prove critical for applications and technologies that require surface displacement of the molecules or knowledge of its temperature-dependent dynamics.