The relationship between the microstructure and hydrogen permeability of the as-cast two-phase Nb-TiNi alloys is investigated and discussed on the basis of the mixing rule. The alloy compositions consisted of the bcc-(Nb, Ti) and B2-TiNi phases are expressed as (Nb4Ti46Ni50)(1-x)(Nb85Ti13Ni2)(x). The alloy for x = 0.19 has a fully eutectic structure of the (Nb, Ti) and TiNi phases in the as-cast state. The hydrogen permeability of this alloy corresponds to that of a model alloy in which these two phases are distributed randomly. The primary TiNi and (Nb, Ti) phases are formed in the alloys for x < 0.19 and x > 0.19, respectively. Their volume fractions decrease and increase with x, respectively. According to the mixing rule, the hydrogen permeability of alloys having a primary (Nb, Ti) phase can be expressed as the model alloy in which the primary phase is isolated in the eutectic structure. However, the hydrogen permeability of alloys having a primary TiNi phase is higher than that expected for the above model alloy. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.