As-cast Nb(19)Ti(40)Ni(41) alloy shows a fully lamellar microstructure, characteristic of the eutectic phase, which turns into a duplex phase by annealing at 1373K for one week. Correspondingly, its hydrogen permeability at 573 K, Phi(573) (K), decreases from 5.7 x 10(-9) to 2.4 x 10(-9) mol H(2) M(-1) s(-1) Pa(-1/2) after annealing. That is, Phi(573) (K) for the annealed sample is 0.42 times that for the as-cast sample. Neither the as-cast nor the annealed alloys are susceptible to hydrogen embrittlement. The hydrogen solubility coefficient (K) and hydrogen content (C) for the annealed Nb19Ti40Ni41 alloy at 573 K is almost the same as those for the as-cast sample, however the hydrogen diffusion coefficient (D) for the annealed sample is 1/3 times that for the as-cast one. The present study demonstrates that lowering of Phi(573) (K) caused by the microstructural change from a lamellar to the duplex microstructure can be attributed to a decrease in D, which is closely related to the change in the (Nb, Ti) phase from a continuous thin layer to discontinuous granules. (C) 2010 Published by Elsevier B.V.