On the basis of ab initio molecular dynamics simulations, we propose that the Fm3̄m symmetry of the high-temperature (above 356 K) phase of Li 2NH results from dynamical disorder due to fast diffusion of Li interstitials and a slower diffusion of Li vacancies. Still, most of the NH groups point tetrahedrally toward the nearest Li vacancy in a geometry reminiscent of the low-temperature Fd3̄m phase. Li diffusion appears at high temperature due to breaking of the tetrahedral clusters of Li interstitials present in our model of the low-temperature Fd3̄m phase. © 2011 American Chemical Society.