The hydrogen permeability of cast Nb30Ti35Ni35-xCox (x = 0...35) alloys is found to increase with the Co content, induced by the tailored microstructures containing high fractions of eutectic and less primary phase. Among the cast alloys, Nb30Ti35Co35 consisting entirely of eutectic exhibits the highest permeability, particularly 2.65 x 10(-8) mol H-2 m(-1) s(-1) Pa-0.5 at 673 K. This permeability is further increased to 3.58 x 10(-8) mol H-2 m(-1) s(-1) Pa-0.5 by aligning eutectic grains perpendicularly to the membrane surface using directional solidification. The high permeability is attributed to the high hydrogen solubility and diffusivity in alloys. Our work demonstrates that hydrogen permeable alloys containing high fractions of eutectic may exhibit high permeability by adequately optimizing morphology, volume fraction and alignment of the bcc-Nb phase in the eutectic as the pathways for hydrogen permeation. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.