KSi silicide can absorb hydrogen to directly form the ternary KSiH₃ hydride. The full structure of α-KSiD₃, which has been solved by using neutron powder diffraction (NPD), shows an unusually short Si-D lengths of 1.47 Å. Through a combination of density functional theory (DFT) calculations and experimental methods, the thermodynamic and structural properties of the KSi/α-KSiH₃ system are determined. This system is able to store 4.3 wt % of hydrogen reversibly within a good P–T window; a 0.1 MPa hydrogen equilibrium pressure can be obtained at around 414K. The DFT calculations and the measurements of hy- drogen equilibrium pressures at different temperatures give similar values for the dehydrogenation enthalpy (≈23 kJ mol⁻¹ H₂) and entropy (≈54 J K⁻¹mol⁻¹ H₂). Owing to its relatively high hydrogen storage capacity and its good thermodynamic values, this KSi/α-KSiH₃ system is a promising candidate for reversible hydrogen storage.