The crystal structure of the ternary imide Li2Ca(NH)(2) has been determined using neutron powder diffraction data on a deuterated sample. The structure consists of infinite layers of edge-shared Ca[NH](6) octahedra, which are separated by Li cations. The mobile Li+ ions in such two-dimensional channels defined by Ca[NH](6) octahedra layers are shown to have a great impact on the hydrogenation properties of the imide. Through detailed structural analysis on the products at various stages of desorption and absorption of the amide-hydride mixture, we proposed a dehydrogenation mechanism involving the mobile small ions in both amide and hydride and a hydrogen storage mechanism for the ternary imide.