The hydrogen absorption-desorption behavior and the crystal structure of Gd2-xMgxNi7 were investigated by pressure composition (PC) isotherm measurements and X-ray diffraction (XRD). Gd1.5Mg0.5Ni7 consists of two phases: 75% Gd2Co7-type structure and 25% PuNi3-type structure. Mg atoms substitute at the Gd site of the MgZn2-type cell in both Gd2Co7-type and PuNi3-type phases. In the hydrogen-free alloy after the PC isotherm measurement, the volumes of the (Gd/Mg)Ni-2 and GdNi5 cells of the Gd2Co7-type phase do not change appreciably, while those of the PuNi3-type phase experience shrinkage and expansion, respectively. A plateau region of Gcl(1.5)Mg(1.5)Ni(7) is observed between 0.06 H/M and 0.82 H/M. The reversible hydrogen content in the absorption desorption cycle is approximately 1.0 H/M. The enthalpy of hydride formation is evaluated as -24 kJ/mol H-2 from the Van't Hoff plot. The absorption desorption behavior is excellent even at low temperatures below 243 K. The hydrogen storage property of Gd1.5Mg1.5Ni7 is markedly improved by Mg substitution. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.