The mechanism of a place-exchange reaction of ligand-protected Au nanoparticles was investigated using diradical disulfide spin labels. Analysis of reaction mixtures using a combination of GPC and EPR allowed us to determine concentration profile and propose a kinetic model for the reaction. In this model, only one branch of the disulfide ligand is adsorbed on the Au surface during exchange; the other branch forms mixed disulfide with the outgoing ligand. The two branches of the disulfide ligand therefore do not adsorb in adjacent positions on the surface of Au nanoparticles; this was ultimately proven by the powder EPR spectra of frozen exchange reaction mixtures. Our data also suggest the presence of different binding sites with different reactivity in the exchange reaction. The most-active sites are likely to be nanoparticle surface defects.