The effect of Mg substitution on Gd4MgNi19 was investigated using X-ray diffraction (XRD) and pressure-composition (P-C) isotherm measurements. Gd4MgNi19 consists of 56% Gd2Co7-type, 27% Ce5Co19-type, and 17% CaCu5-type structures, which are retained in the hydride phase. Peak broadening was observed in the XRD profile of the hydride phase, while the degree of peak broadening decreased in the hydrogen-desorbed alloy after the PC isotherm measurement. The reversible hydrogen capacity reached 1.0 HIM. There was a wide plateau region between 0.10 H/M and 0.80 H/M during the absorption-desorption process. Gd5Ni19 consists of 89% Sm5Co19-type and 11% CaCu5-type structures. After the P-C isotherm measurement, the hydrogen-desorbed Gd5Ni19 alloy showed severe -peak broadening in the XRD profile. The reversible hydrogen capacity was 0.80 H/M, and there was a sloping plateau in the P-C isotherm. Mg substitution decreases the severe peak broadening and lattice strain after the P-C isotherm measurement, and enhances the hydrogenation property. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.