Gold nanocrystal superlattices were synthesized in colloidal form by ligating the nanocrystal surface with dodecanethiol ligand. Formation and dissolution of the superlattices in the colloid were studied by UV-vis spectroscopy and dynamic light scattering. The phase boundary between nanocrystals and their superlattices in solution was found to be similar to the case of macroscopic crystals made of atoms or molecules. Lowering the temperature across the phase boundary resulted in nucleation of nanocrystals into superlattices and the superlattice size depended on the rate of nucleation. UV-vis spectroscopy and dynamic light scattering also showed that thermal dissolution of the superlattices occurred after the temperature of the colloid was raised across the phase boundary.