High-temperature, high-pressure hydrogen attack is a degradation mechanism that threatens the integrity of critical components in re fineries and petrochemical plants. This phenomenon was discovered in the early twentieth century, however, the experience-based recommended practice used to design for such applications is still being corrected due to ongoing failures in industry. Research on the topic, in the late twentieth century, focused on experimenting with hydrogen attack. Since then, most of the work has been focused on the modelling aspect of the issue. The aim of the work presented in this thesis was to understand the mechanism of hydrogen attack, using experimental methods, in order to reach a mitigation solution that utilises current literature and improves upon the recommended practice. Hydrogen attack resistance was examined in the commercially-used steel, 2(1/4)Cr-1Mo, revealing the importance of heat treatment conditions to bring the precipitated carbides closer to their equilibrium state, which decreases carbon activity in the steel and consequently increases the resistance to reaction with hydrogen. For the first time, experimental methods, such as quantification of carbide volume fractions using synchrotron X-ray diffraction as well as optical microscopy to measure void volume-fractions, were used in characterising hydrogen-attack resistance. Consistent interpretations of experimental data have been obtained by comparison against equilibrium calculations, microstructure and synchrotron X-ray diffraction. Some novel approaches were also adopted for the first time. Interphase precipitation in vanadium steel was shown in this work as a solution to mitigate hydrogen attack at the same exposure conditions where some samples of 2(1\4)Cr-1Mo steel failed. The mitigation was explained by the direct precipitation of stable vanadium carbides during the austenite to ferrite transformation, which immediately depletes ferrite of carbon, thus reducing the carbon activity. This is not the case in tempered martensite that contains many lattice defects, where excess carbon may be retained, hence increasing carbon availability for methane formation. Vanadium carbides are found to be more stable than chromium or molybdenum carbides in the alloy systems considered. The hydrogen attack mechanism leads to bubble formation with these bubbles progressively linking by a creep mechanism, until ultimate failure. Vanadium is generally known to particularly enhance the creep strength, so any voids created would have a difficulty in linking up by creep deformation in the presence of carbides. Another novel approach was to use the recently invented HT10 steel, that has been shown to outperform 2(1/4)Cr-1Mo steel in terms of ambient mechanical properties and low-temperature hydrogen embrittlement. The work in this thesis has shown, for the first time, that HT10 outperforms 2(1/4)Cr-1Mo in terms of elevated-temperature hydrogen attack caused by methane formation. Given that HT10 is similar in many aspects to the commercially-used steel in hydrogen-attack applications (2(1/4)Cr-1Mo), HT10 steel can o er a suitable replacement more easily than the vanadium steel studied that still requires further investigations in terms of mechanical properties. The thesis fi nishes with the definition of an interesting programme of future work that not only can reveal fundamental knowledge never before attempted, but also may lead to technological exploitation.