The microstructural changes during hydrogen absorption desorption cycles of an A(2)B(7)based La-Mg-Ni alloy with a nominal composition of La1.5Mg0.5Ni7.0 were systematically investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The ternary La-Mg-Ni alloy was mostly composed of 2H-A(2)B(7) phase with minor inclusions of 3R-A(5)B(19), 2H-A(3)B(19) and 3R-AB(3) phases existing as parts of intergrowth structures with the major A(2)B(7) phase. Most parts of the major 2H-A(2)B(7) phase containing Mg exhibited an excellent crystal structure retention after the hydrogen absorption desorption cycles at 80 degrees C. Two types of defected bands were found to develop after the first hydrogen absorption desorption cycle. The first ones are amorphous bands developed inside the minor 3R-AB(3) phase, while the second ones develop as heterogeneously strained regions inside the major 2H-A(2)B(7) phase. Both the defected bands are considered to be responsible for the irreversible hydrogen capacity of the A(2)B(7)-based La1.5Mg0.5Ni7.0 alloy during the hydrogen absorption desorption cycles at 80 degrees C. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.