In order to develop alloys combing high hydrogen permeability with large resistance to the hydrogen embrittlement, microstructures and hydrogen permeability (Φ) have been investigated for the as-cast alloys on the straight line connecting the eutectic {TiCo + (Nb, Ti)} phase and the Nb-rich primary (Nb, Ti) phase in the Nb–Ti–Co system. The alloys on the above-mentioned line consist of the TiCo compound and the (Nb, Ti) solid solution. The value of Φ increases with increasing temperature and volume fraction of the primary (Nb, Ti) phase. The most Nb-rich Nb60Ti21Co19 alloy shows the highest Φ value of 3.99 × 10−8 (mol H2m−1 s−1 Pa−0.5) at 673 K, which is 2.6 times higher than that of pure Pd. The present work demonstrates that highly hydrogen permeable alloys are obtainable in the Nb rich Nb–Ti–Co ones on the straight line connecting the eutectic {TiCo + (Nb, Ti)} phase and the Nb-rich primary (Nb, Ti) phase.Highlights▶ The alloys on the line connecting the eutectic and the (Nb, Ti) phases. ▶ These alloys consist of the TiCo compound and the (Nb, Ti) solid solution. ▶ The hydrogen permeability increases with the volume fraction of (Nb, Ti) phase. ▶ The most Nb-rich Nb60Ti21Co19 alloy shows the highest hydrogen permeability.