Nanometre-scale gold particles are currently intensively investigated for possible applications as catalysts, sensors, biolabels, drug delivery devices, biological contrast agents and as components in photonics and molecular electronics. The particles are prepared in a solution from gold salts and their reactive gold cores can be stabilized with various organic ligands. Particularly stable particles can be synthesized by using organothiol ligands that have a strong chemical interaction to gold, producing precise compositions in the size range of 1 to 3 nanometres. Modification of the protecting molecular overlayer is a key step in almost all applications. A detailed structural atomistic understanding of the processes of the exchange reaction has been lacking.