Keywords:Ni-Base AlloysGrain Boundary Character CarbidesCrackingHydrogen embrittlement (HE) of Ni-base alloys is an important concern for sour well applications and is very sensitive to microstructure. We show that fracture in tensile tests of H-charged alloy 725 is intergranular and that grain boundaries (GBs) rather than carbides play the key role in crack initiation and propagation. We assess the GB character using electron backscatter diffraction and perform tensile testing in situ in a scanning electron microscope allowing for the clear identification of the GBs along which cracks initiate and propagate. By designing our experiment to yield copious secondary cracks we study a statistically meaningful number of the cracking events providing the ability to determine GB types that are most amenable to crack initiation and propagation. The utility of these results for evaluating and selecting materials on the basis of their HE-resistance and for the design of HE-resistant microstructures will be discussed.