We investigated the crystal structure and cyclic hydrogen absorption-desorption properties of Pr2MgNi9. The structural model is based on the PuNi3-type structure; the Mg atom is assumed to substitute for the Pr site in an MgZn2-type cell. The refined lattice parameters were determined from X-ray diffraction. A wide plateau region was observed in the P-C (pressure composition) isotherm at 298 K. The maximum hydrogen capacity reached 1.12 HIM (1.62 mass%) under a hydrogen pressure of 2.0 MPa. After 1000 hydrogen absorption-desorption cycles, the hydrogen capacity was superior to that of LaNi5 (82%). Anisotropic lattice strain occurred in the hydriding process. The anisotropic peak-broadening vector was determined to be < 001 >. The calculated anisotropic lattice strains of the initial cycle and after 1000 cycles were far smaller than those of LaNi5. Copyright (c) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.