Research highlights ▶ The new phase Li 2MgPt can be formed by mechanical milling of the constituent elements. ▶ Li 2MgPt crystallizes in the cubic P 4 ¯ 3 m space group and is isostructural to the known phases Li 2MgSi and Li 2MgIr. ▶ Hydriding of Li 2MgPt proceeds according to the reaction Li 2MgPt + H 2 → MgPt + 2LiH. Tetragonal MgPt is a new member of the Mg–Pt binary phase diagram previously misidentified by others as an Mg–Pt–C–H quaternary. ▶ Li 2MgPt can also be formed by heat treating a mixture of LiH and MgPt in a 2:1 ratio. The reaction occurs below the decomposition temperature of LiH, so that MgPt can be considered to destabilize LiH. ▶ Even ignoring the cost of Pt, this system is unsuitable for cyclable hydrogen due to the accumulation of undesirable secondary phases with repeated exposure.