V-based alloy membranes with the body-centred-cubic structure are of great interest for hydrogen separation applications due to their low cost and high permeability. As microstructure can greatly influence membrane performance, internal microstructures resulting from different processing conditions, and their effects on hydrogen permeability, have been investigated for the V-15Ni (wt%) BCC alloy. The initial coarse-grained, as-cast microstructure evolved into a fibrous/lamellar microstructure with a small grain size during cold-rolling deformation, and a significant reduction in hydrogen permeability accompanied this deformation. Subsequent annealing decreased the defect density and increased the grain size and hydrogen permeability. These results show that, aside from compositional optimization of these BCC alloys to minimize the effects of hydrogen embrittlement, the control of microstructural defects is central to the development of high-permeability alloy membranes. (C) 2010 Published by Elsevier B.V.