For pure hydrogen separation, palladium was deposited on surface-treated polybenzimidazole (PBI-HFA, 4,4'-(hexafluroisopropylidene)bis(benzoic acid)) via the vacuum electroless plating technique (VELP). Since the hydrophobic characteristics of the polymer surface restrict strong adhesion of Pd on it and cause the peel-off of Pd film, various surface treatments have been employed. To increase the number of Pd anchoring sites on the PBI-HFA surface, mechanical abrasion (polishing) was applied, and to increase the hydrophilicity of the PBI-HFA surface, wet-chemical and O-2 plasma treatment (dry etching) were used. The thickness and effective permeating area of the deposited Pd films on the PBI-HFA membranes were estimated to be in the range of 160-340 nm and 8.3 cm(2), respectively. Among the tested membranes, membranes with Pd layers deposited on O-2 plasma treated PBI-HFA surfaces had the most uniform microstructure and the least number of defects compared to the other membranes. Gas permeation experiments were performed as a function of temperature and pressure. The gases used in the permeation measurements were H-2, N-2, CO2, and CO (99.9% purity). A Pd-O(2)30 m membrane, fabricated by O-2 plasma surface treatment during 30 min, exhibited superior gas separation performance (H-2 permeability of 275.5 Barrer), and proved to be impermeable to carbon monoxide. Enhancement of H-2 permselectivity of Pd/PBI-HFA composite membrane treated by O-2 plasma shows promising hydrogen separation membrane. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.