The crystal structure of a mixed amide-imide phase, RbMgND2ND, has been solved in the orthorhombic space group Pnma (a = 9.55256(31), b = 3.70772(11) and c = 10.08308(32) Å). A new metal amidehydride solid solution, Rb(NH2)xH(1−x), has been isolated and characterized in the entire compositional range. The profound analogies, as well as the subtle differences, with the crystal chemistry of KMgND2ND and K(NH2)xH1−x are thoroughly discussed. This approach suggests that the comparable performances obtained using K- and Rb-based additives for the Mg(NH2)2-2LiH and 2LiNH2− MgH2 hydrogen storage systems are likely to depend on the structural similarities of possible reaction products and intermediates.