jats:titleAbstract</jats:title>jats:pHigh temperature hydrogen attack is a damage mechanism that occurs in critical steel components in petrochemical plants and refineries when the hydrogen penetrates the steel and reacts with the carbides within to produce pores containing methane. With the motivation of understanding the role of carbide stability on the reaction with hydrogen, samples of a classic 2jats:inline-formulajats:alternativesjats:tex-math$$\frac{1}{4}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:mfrac mml:mn1</mml:mn> mml:mn4</mml:mn> </mml:mfrac> </mml:math></jats:alternatives></jats:inline-formula>Cr-1Mo steel were subjected to a variety of heat treatments that generate a corresponding variety of precipitates, prior to exposure to high-pressure hydrogen in an autoclave. Using quantitative carbide, porosity and microstructural characterisation, it has been possible to demonstrate the roles of four variables: (a) the carbon residue present in the ferrite; (b) the non-equilibrium chemical composition of carbide; (c) the fraction of the carbide that is closest to the thermodynamic equilibrium state and (d) the initial microstructural state, i.e., whether it is martensitic or bainitic prior to heat treatment.</jats:p>