The existence of Li-rich super-lightweight intermetallics in the Li-Mg-Si ternary system has attracted attention for high capacity hydrogen storage materials. The hydrogenation properties of the alloys were studied by thermogravimetric analysis, differential scanning calorimetry in H-2 atmosphere and X-ray diffraction. The Li-rich alloy absorbs the highest amount of hydrogen (8.8% w/w for Li70Mg10Si20), while the Mg-rich alloy (Li30Mg40Si30) absorbs 6.0% w/w H-2 and shows the first experimental evidence for LiMgH3 formation with LiNbO3-type structure during hydrogenation. The extension of homogeneity range of existing phases from Li-Mg-Si system was found and the crystal structures of four ternary phases, Li2+xMg1-xSi (x = 0.268), Li65-x+yMg30+x-ySi33 (X = 9.15, y = 1.23), Li7-x+yMg5+x-ySi4 (x = 1.14, y = 0.61) and Lix+yMg2-xSi (x = 0.51, y = 0.39), were solved and more precise refined using X-ray single crystal diffraction data. Electronic structure calculations reveal an increased occupation of electronic states at the Fermi level for Li12+xMg3-xSi4 in comparison to Li2+xMg1-xSi. The results of crystal orbital Hamilton population (COHP) and integrated crystal orbital Hamilton population (iCOHP) calculations for both structures indicate the strongest interactions between Mg-Si (-iCOHP = 1.999 eV) and the weakest between Li-Li (-iCOHP = 0.049 eV). Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.