The effects of catalyst CoS2 on the microstructure and gaseous hydrogen storage behaviors of Sm5Mg41 + x wt% CoS2 (x = 0, 5, 10) alloys prepared by milling CoS2 nanoparticles and the as-cast Sm5Mg41 alloy have been investigated. The alloys before hydrogenation are composed of Sm5Mg41 and SmMg3 phases; ball milling refines the crystal grain. The Sm3H7 and MgH2 phases appear after hydrogenation, furthermore, the Mg phase is formed and the Sm3H7 phase is retained after dehydrogenation. The CoS2 phase always exists in the form of nanoparticles embedded into the surface of the catalyzed alloy, which mainly presents a nanostructure containing some crystal defects, such as dislocations, grain boundaries and twins. These microstructures play a beneficial role in reducing the total potential barrier that the hydrogen absorption/desorption reaction must overcome, hence improving the hydrogen storage kinetics of the alloys. The dehydriding activation energy of the alloys is 128.19, 101.67, and 95.49 kJ mol(-1) H-2, and the hydrogenation enthalpy of the alloys is -81.72, -80.65, and -79.28 kJ mol(-1) H-2 with x = 0, 5, and 10, respectively. Therefore, the addition of the CoS2 catalyst significantly improves the hydrogen storage kinetics but slightly reduces the stable thermodynamics of the hydrides. (C) 2017 Elsevier Ltd. All rights reserved.