In situ neutron diffraction is for the first time demonstrated to be a powerful tool for precisely following the structural changes associated with mobile light ions during Li⁺/Na⁺ ion exchange.  While ion exchange is traditionally accomplished using a liquid electrolyte, the present experiments were facilitated by the use of non-molten salts as the source of ions.  A novel isotropic Li-ion conductor was investigated in the present work. High quality structural data could be obtained through the Rietveld refinement of neutron diffraction data collected with ~30 minute time steps.  The mechanism of ion exchange could be readily resolved from this data, including the identification of a hidden phase transition not apparent from studies of the lattice parameters along, the identification of site-specific contributions to the overall volume change during ion exchange, and the observation of different equilibrium ratios of phases at different temperatures.  This method should have general applicability to the study of ion exchange processes in solids, but provides the most advantages for the study of oxide ceramics due to the high sensitivity of neutrons to oxygen and other light elements.