Hydrogen permeation tests were carried out on ultrafine-grained palladium processed by high-pressure torsion (HPT) and the results were compared with those on an annealed coarse-grained state. It is shown that hydrogen diffusion is similar above 200 degrees C but is enhanced at temperatures below 200 degrees C in the ultrafine-grained state. All samples were subjected to X-ray diffraction analyses after the hydrogen permeation tests. It is shown that hydride formation occurs at lower temperatures in both coarse- and ultrafine-grained states but the hydride formation temperature is reduced to a lower temperature in the ultrafine-grained state. This study suggests that grain boundaries of palladium act as diffusion pass but not as the sites for hydride formation. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.