Changes in microstructures and hydrogen permeability by long-time annealing (168 h) at 1073, 1173 and 1173 K for eutectic Nb30Hf35Co35 alloy are investigated. After annealing at 1073 K and 1173 K for 168 h, the fully lamellar eutectic microstructure in as-cast state has disappeared and turned to a duplex microstructure. Correspondingly, the hydrogen permeability at 673 K, decreases from 3.3 x 10(-8) to (0.82-1.5) x 10-8 mol H-2 m(-1) s(-1) Pa-0.5 after annealing. The annealed (a-) sample at 1273 K for 168 his brittle and susceptible to hydrogen embrittlement that results from the appearance of the additional Hf2Co phase. The hydrogen solubility coefficient (K) for each a- samples at 673 K is almost the same as that for the as-cast sample, while the hydrogen diffusion coefficient (D) for the a- samples are (1/4-1/2) times than that for the as-cast one. It is thus concluded that the lower permeability by annealing for eutectic Nb30Hf35Co35 is mainly attributed to the significant reduction of hydrogen diffusivity after annealing. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.