The synthesis, thermodynamic destabilisation and hydrogen absorption/desorption characteristics of the LixNa1-xMgH3 system with (x = 0, 0.2, 0.5 and 0.8 M ratios) have been investigated. Samples were mechanically milled under argon for 5 h; then characterised by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Diffraction peaks of NaMgH3 phase shifted to higher angles and lattice parameters decreased due to the Li addition into the system. 2 and 3 endothermic reactions were observed for the Li-x substituted samples (x = 0, 0.2, 0.5, 0.8). Li0.8Na0.2MgH3 hydride showed the best performance among the other quaternary hydrides (synthesised in this work) releasing 5.2 wt.% of H-2 at 314 degrees C. Rehydrogenation of the decomposed LixNa1-xMgH3 (x = 0, 0.2, 0.5 & 0.8) samples was experimentally confirmed under 10 bar H-2 at -250 degrees C. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.