Abstract Ni 3Al alloys, of nominal composition Ni-24 at.% Al and three different B concentrations (50, 100 and 500 wppm), were tensile tested at room temperature in high-purity H 2 gas at pressures ranging from ∼6 × 10 −8 to 7 × 10 3 Pa. The highest elongations to fracture were obtained in ultrahigh vacuum: ∼36, 45 and 60% for the 50, 100 and 500 wppm B alloys, respectively. With increasing H 2 pressure, the ductility of all three alloys dropped precipitously. Accompanying this drop in ductility was a change in the fracture mode from predominantly transgranular to predominantly intergranular. An intriguing result of our present study is that, at the higher H 2 pressures employed, B-doped Ni 3Al (50 or 100 wppm) is brittle and fractures predominantly intergranularly, whereas B-free Ni 3Al is ductile and fractures predominantly transgranularly. This result indicates that B—by possibly promoting the dissociation of molecular H 2 into atomic H— embrittles Ni 3Al in a dry H 2 environment, unlike in H 2O-containing environments, where B suppresses grain boundary fracture and improves the ductility of Ni 3Al. © 1997 Acta Metallurgica Inc.