The project seeks to establish a fundamental understanding of the effect of inhibitors in high-pressure gaseous hydrogen on the detrimental interaction between hydrogen and steels. This knowledge is essential for advancing a sustainable energy system, which depends on safe transport and storage of hydrogen-containing gas mixtures in existing or new high-pressure structural networks. Gas impurities added to pressured hydrogen can have an inhibiting effect on hydrogen uptake in the material, mitigating hydrogen embrittlement. First, a reproducible in-situ charging methodology for gaseous hydrogen with or without gas impurities was developed and validated for both bcc (body centered cubic) and fcc (face centered cubic) materials. The research then focused on systematically evaluating the role of O₂ as an inhibitor of hydrogen uptake in various pipeline steels with differing chemical compositions and microstructures. The efficiency of the inhibitor was evaluated at different concentrations and exposure times using melt extraction, which determines the hydrogen uptake capacity. Ex-situ tensile tests on gaseously pre-charged samples were conducted to qualitatively rank materials according to their hydrogen embrittlement susceptibility and to examine the impact of hydrogen on fracture surfaces. In addition, a novel in-situ wedge opening load set-up in a gaseous hydrogen environment is developed. This approach enables quantitative assessment of inhibitor effects on the hydrogen embrittlement behavior by measuring crack extension and determining the influence of hydrogen on the critical stress intensity factor.