This research focused on a comprehensive microstructural and mechanical property characterisation study of the Ni-Fe-Cr alloys 718 and 945X. The aim of the project was to better understand the relationship between performance and microstructure of existing (Alloy 718) and newly developed (Alloy 945X) high strength nickel alloys focusing on downhole applications. The main difference between the two alloys is that alloy 945X has lower Nb content than alloy 718, which may minimise the tendency to form delta when combined with correct processing. Previous studies have related the hydrogen embrittlement in alloy 718 with the collection of hydrogen by delta phase.Microstructural characterisation of the new alloy 945X after long term isothermal exposure up to 120 hours in the temperature range 650◦C to 900◦C was conducted with scanning electron microscopy (SEM), to generate a time-temperature-transformation (TTT) diagram. The TTT diagram was used as a road map for designing two isothermal heat treatments of alloy 945X on tensile specimens. Then, the effect of hydrogen charging on the tensile properties and microstructure of the 'as-received' and these two variant heat treatments was investigated. Fractographic analysis showed that, in the presence of hydrogen, intergranular fracture occurred for all the heat treatments, regardless the presence of delta phase at grain boundaries. There was no simple correlation between the volume fraction of delta-phase and susceptibility to hydrogen assisted embrittlement. Rather, it was demonstrated that the morphology and distribution of delta-phase along grain boundaries plays a key role and the other precipitate phases also have an influence through their effect on the ease of strain localisation.This study also examined the hydrogen embrittlement sensitivity of nickel alloy 718 given four different heat treatments to obtain various microstructural states. Each heat treatment leads to differences in the precipitate morphologies of γ', γ'' and delta phases. Material characterisation and fractography of the examined heat treatments were performed using a high resolution FEG-SEM. Three specimens of each condition were pre- charged with hydrogen and tensile properties were compared with those of non-charged specimens. It was observed that hydrogen embrittlement was associated with intergranular and transgranular microcrack formation, leading to an intergranular brittle fracture. delta phase may assist the intergranular crack propagation, and this was shown to be particularly true when this phase is coarse enough to produce crack initia- tion, but this is not the only factor determining embrittlement. Other microstructural features play a role, as does the strength of the material.Finally, the evolution of delta-(Ni3Nb) phase in alloy 718 from the early stages of precipitation, with a particular focus on identifying the grain boundary characteristics that favour precipitation of grain boundary delta phase was investigated. Results showed that delta phase was firstly formed on Σ3 boundaries after 5 hours at the examined temperature (800◦C). Increasing ageing time at 800◦C was observed to lead to an increase in size and precipitation of phases γ'-γ''-delta, an increase in fraction of the special CSL boundaries and an evolution in the morphology of twins and the growth of grains.