The hydrogen storage properties of ErNi5-xAlx (x = 0, 0.5, 0.75, 1, 1.25, and 1.5) alloys were investigated by pressure-composition isotherms and in situ X-ray diffraction measure-ments under a hydrogen atmosphere. Catalytic reactivities toward the hydrogenation of alkynes (acetylene and propyne) over ErNi5-xAlx (x = 0, 1, and 1.5) alloys were also studied and the contribution of absorbed hydrogen to hydrogenation is discussed. All ErNi5-xAlx alloys possess a hexagonal structure (CaCus-type) with the space group P6/mmm. The substitution of Al for Ni facilitated hydrogen absorption at lower hydrogen pressures by the formation of larger interstitial spaces. ErNi3.5Al1. 5Hn with absorbed hydrogen showed higher reactivities for the catalytic hydrogenation of acetylene and propyne than ErNis and ErNi4Al without absorbed hydrogen. The reason for this was concluded to be that absorbed hydrogen activates adsorbates (acetylene and hydrogen) that are supplied from the gas phase. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.