The present study encompasses the results of a comprehensive performance assessment of different pilot-scale Pd/Alloy composite asymmetric membranes under long-term coal-derived syngas atmosphere testing. The membranes were developed and tested first at the CIMS-WPI laboratory under pure H-2 and later at NCCC under a mixture of N-2/H-2 and H-2 enriched syngas coming from a TRIG (TM) gasification system. Thirteen membranes were examined for 4275 h and classified in four types: Pd, Pd/Au, Pd/Pt and Pd/Au/Pt membranes. Their respective H-2 permeance and purity profiles, permeance difference under pure hydrogen and syngas conditions as well as stability of operation were assessed. The membranes showed good stability in syngas although exhibiting an initial drop in permeance when compared to that under pure hydrogen. Pure Pd membranes displayed the highest permeability, but showed pinholes after being exposed to syngas. Pd/Pt membranes showed a reduced permeance drop under syngas conditions, indicating sulfur resistance properties. Furthermore, Pd/Au/Pt membranes, presented for the first time in the literature, displayed stable permeance and purity profile. The inhibition of the initial permeance drop and the long-term operation of these ternary membranes were demonstrated. Finally, Pd/Au membranes were shown to display the most stable permeance and purity characteristics. In this case, the initial permeance drop was significantly reduced, exhibiting the lowest decline amongst all cases considered. It is hypothesized that Au may act as a patch paste blocking defects on the Pd layer. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.