Nanometer-sized Mg hydride clusters may form in a self-organized way by the hydrogenation of a nonequilibrium Mg-Ti alloy. Here the Mg hydride is destabilized by the interface energy between the two metal hydrides. To obtain an even more destabilized Mg hydride, we increased the interface energy by the addition of Cr, which is immiscible with Mg as Ti. Indeed, Mg layers surrounded by Ti-Cr layers show hydrogen plateau pressures higher than when Mg is surrounded by Ti. Destabilization of Mg hydride is also observed in hydrogenated Mg-Ti-Cr thin-film alloys, resulting in hydrogenation plateaus flatter and at higher pressures than in hydrogenated Mg-Ti thin film alloys. Our results suggest that by screening alloys on the basis of their immiscibility with Mg, we can tune the thermodynamics and kinetics of hydrogen absorption and desorption in Mg-H. This concept paves the way for the development of light-weight and cheap Mg-based functional materials in the metal-hydrogen system.