The membrane selectivity is among the key determinants that affect the overall H₂ separation efficiency and the long-term stability of the composite Pd and Pd/alloy membranes. To mitigate high temperature leak growth and to improve the long-term stability of the membrane selectivity, a novel synthesis procedure was implemented during the preparation of the composite Pd membrane supported on a media-grade 0.1 µm Inconel support, which was oxidized at 800°C for 12 hours to generate the intermetallic diffusion barrier. The successful synthesis of the 7.6 µm thick pure-Pd/Inconel membrane with a special emphasis on the formation of leak resistant deposition layers and saturated (stable) grain boundaries, led to an excellent long-term stability at 450°C with essentially infinite H₂/He ideal selectivity. At 450°C and at a ΔP of 35 psi (P_High=50 psia & P_Low=15 psia), the He leak of the membrane was undetectable and the membrane maintained an excellent H₂/He selectivity stability over a period of ~845 hours with numerous H₂/He cyclings. Upon the completion of the long-term H₂ permeation characterization (~1250 hours within 300-450°C), the high pressure testing of the 7.6 µm Pd /Inconel membrane was conducted at a feed side pressure as high as 116 psia (7.9 bara). At 400°C and a ΔP of 101 psi (P_High=116 psia & P_Low=15 psia), the stable H₂ flux for the 7.6 µm Pd/Inconel membrane was as high as 150 scfh/ft², exceeding the 2007 DOE target under similar operating conditions. Furthermore, the He leak after the high-pressure testing remained undetectable. The long-term selectivity stability is a significant achievement towards the successful utilization of Pd- and/or Pd/alloy-based catalytic membrane reactors for the production of high-purity H₂ suitable for fuel cell applications in industrial steam reformers and/or high temperature water-gas shift reactors integrated into the downstream of the industrial coal gasification units.