TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Composition: The alloy comprises (in %): palladium (35-55, preferably 47), the additive metal (X) (1-15, preferably 7), and copper (53, preferably 46). The atomic size of (X) is less than 16% with respect to palladium and copper. The alloy is prepared by selecting a base formulation of palladium-copper determined to be optimal for high permeability from the data, and substituting copper with the selected metal for a limited portion to enhance the BCC stability without compromising the permeability, to enhance BCC permeability without compromising the thermal and chemical stability, and to enhance the thermal and chemical stability without compromising the permeability. Preferred Process: The selecting step of the most preferred elements comprises calculating the heat of formation for each alloy resulting by substituting elements from the group of preferred elements using a density functional theory atomic scale simulation program, and selecting the elements that yield the alloys having the lowest heat of formation or suitable for the representative operating environment. The application of Hume-Rothery rules to the elements replaces a Hume-Rothery rule regarding atomic size difference less than 15% with an analysis of the lattice size difference. The application of Hume-Rothery rules comprises determining the binary pair of palladium and copper i.e. partly substituted with the selected element, evaluating each of the elements against the obtained determined results, and determining the magnitude of lattice size difference. Preferred Components: The metal is niobium, tantalum (both preferred), vanadium, iron, chromium, molybdenum or tungsten. (X) further comprises an additional metal i.e. non-BCC and that enhances a further required property of the membrane in addition to thermal stability. The additional metal is face centered cubic structure (FCC) and enhances the ductility of the membrane. The additional metal is gold, silver, rhenium, ruthenium, rhodium, yttrium, cerium, nickel, iridium, platinum, cobalt, lanthanum or indium. EXAMPLE - Typical composition comprised (in wt.%): palladium (47.9); copper (37.9); and tantalum (14.2).