Structural changes in multiphase La0.8Mg0.2Ni3.5 alloy during hydrogen absorption/desorption cycles were studied in this work. XRD and TEM characterizations confirm that transformation from crystallinity to amorphous viz, hydrogen induced amorphization (HIA) of both (La,Mg)(2)Ni-7 and (La,Mg)(5)Ni-19 phases has occurred after 100 repeated cycles. In addition, P-C-T plateau is found to become slope and narrow, indicating that degradation of the hydrogen storage capacity comes with partial amorphization in the cyclic process. In order to comprehend the mechanism of HIA in the La-Mg-Ni compounds, hydrogenation processes from low to high hydrogen pressure at various temperatures were adopted and the structural evolution was investigated. Two kinds of crystalline La-Mg-Ni based hydrides are found to present successively, but transformation to amorphous subsequently takes place with the hydrogen concentration increased. Structural changes of the alloy are almost the same and no disproportion of the La-Mg-Ni phases is detected when hydrogenated at both the room and elevated temperatures. However, amorphization of the La-Mg-Ni phases becomes more severe as the charging temperature decreased. HIA of the La-Mg-Ni phases is thus considered to be non-thermally activated, which is launched directly by distortion of the lattice with hydrogen concentration increased. Furthermore, it is established that HIA of (La,Mg)(2)Ni-7 phase is more liable to occur but hard to recrystallize compared with that of (La,Mg)(5)Ni-19 compound. The structural stability towards HIA of various compounds in the multiphase La0.8Mg0.2Ni3.5 alloy is then believed to be as follows: LaNi5 > (La,Mg)(5)Ni-19 > (La,Mg)(2)Ni-7. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.