Abstract: The disclosure provides an H2 separation membrane comprised of an alloy having the composition Cu(100-x-y)PdxMy, where x is from about 35 to about 50 atomic percent and where y is from greater than 0 to about 20 atomic percent, and where M consists of magnesium, yttrium, aluminum, titanium, lanthanum, or combinations thereof. The M elements act as strong stabilizers for the B2 phase of the alloy, and extend the critical temperature of the alloy for a given hydrogen concentration and pressure. Due to the phase stabilization and the greater temperature range over which a B2 phase can be maintained, the alloy is well suited for service as a H2 separation membrane, particularly when applicable conditions are established or cycled above about 600° C. over the course of expected operations. In certain embodiments, the B2 phase comprises at least 60 estimated volume percent of the alloy at a steady-state temperature of 400° C. The B2 phase stability is experimentally validated through HT-XRD.