The structure and phase variation of Ca5Si3 upon hydrogenation were systematically investigated using combined neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), and first-principles calculations. The hydrogen absorption equilibrium was first attained with formation of Ca5Si3H(D)(0.53) (I4/mcm) with H exclusively located in Ca-4-tetrahedral sites. More hydrogen absorbed into the system under higher pressure leads to dissociations into CaH2 (an amorphous hydride at higher pressures) and CaSi. The hydrogen-induced formation of an amorphous phase under higher pressures is very unusual in Cr5B3-type compounds and the observed formation of CaH2 upon hydrogen absorption confirmed the proposed composition equilibrium between A(5)Tt(3) (A = Ca, Sr; Tt = Si, Ge, Sn) and AH(2). (c) 2008 Elsevier B. V. All rights reserved.