The mechanical behaviour of L1 2 -type Ni 3 (Si,Ti) alloys has been observed to depend on the environmental medium, strain rate, microstructure, pre-deformation, alloy composition and residual hydrogen content. It is shown that hydrogen released from the environment as well as residual hydrogen in the material interacts with lattice defects such as grain boundaries, dislocations and vacancies, and affects the tensile ductility and fracture behaviour of Ni 3 (Si,Ti) alloys. Grain boundaries offer sites for hydrogen condensation and cause embrittlement due to intergranular fracture, whereas dislocations and vacancies can trap hydrogen, thereby suppressing intergranular fracture and associated embrittlement.