NOVELTY - Palladium based alloy as a hydrogen-separation membrane, comprises palladium, copper and an additive metal (X), where (X) is a metal i.e. body centered cubic structure (BCC) and that stabilizes the beta-BCC phase in the alloy that any non-BCC type phases present in the alloy have negligible effect on temperature-related phase changes of the alloy.    USE - The alloy is useful as the hydrogen-separation membrane in the fuel processor (claimed) e.g. hydrogen generator.    ADVANTAGE - The alloy is stable and resistant to sulfur-contamination for 1000 hours in the presence of sulfur-containing gases, and to carbon monoxide for 1000 hours in the presence of a water-containing reformate (claimed).    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) an alloy of formula Pd-Cu-X that acts as the hydrogen-separation membrane in the presence of sulfur-containing reformate, where the alloy is stable and resistant to sulfur-contamination for 1000 hours in the presence of sulfur-containing gases having a ratio of water to sulfur of 5000:1 and a temperature of 250degreesC, and is stable and resistant for 1000 hours to carbon monoxide in the presence of a water-containing reformate, where the ratio of water to carbon monoxide is at least 2:1 and the temperature is 250degreesC;    (2) a fuel processing system (10) for separating hydrogen from process stream (18) containing hydrogen comprises a first chamber (16) to receive the process stream, a second chamber (22) to receive hydrogen separated from the process stream in the first chamber, a hydrogen-separation membrane (12) separating the first and second chambers for the selective separation of hydrogen from the process stream in the first chamber, where the hydrogen-separation membrane comprising the alloy; and    (3) a method of selecting at least one element for combination with palladium and copper to obtain the alloy comprising the 'X', applying Hume-Rothery rules to select 'X', and selecting the most preferred elements that optimize for the alloy.    DESCRIPTION OF DRAWING(S) - The figure shows a schematical representation of the hydrogen generator:    Hydrogen generator; (10)    Hydrogen-containing process stream; (18)    Hydrogen receiving chamber; (16, 22)    Hydrogen-separation membrane. (12)