Abstract In both its upstream and downstream sectors, the oil & gas industry has to cope with hydrogen cracking in various hydrogenating media, including sour service, cathodic protection (CP), anhydrous HF, or cold hydrogen gas. In order to allow the general transposition of the detailed materials specifications recently established for wet H2S, it has been attempted to directly compare the severity of these different media. The principle of the method employed is to measure the back pressure PH2 developed in a hollow sensor which can be immersed in any corrosive medium. In the steady state, PH2 is in solubility equilibrium with the H content of the steel. Its variations can therefore be used for characterizing the actual level of H charging. Measurements are made under CP and in the upper part of the EFC 16 diagram. Surprisingly, it was found that PH2 depends mainly on pH, and much less on the partial pressure PH2S. The results also show that hydrogen effusion towards the corrosive medium is possible during charging, and in the presence of H2S. This is in contradiction with the hitherto commonly accepted mechanism of hydrogen charging. In fact, the severity of SSC also appears to be related to the kinetics of hydrogen charging, rather than to the steady state hydrogen level. Again, this is inconsistent with the conventionally acknowledged metallurgical process. Two new mechanisms are therefore proposed, for both hydrogen charging in the presence of H2S, and hydrogen stress cracking: In sour service, hydrogen uptake is considered to involve direct deprotonation of the various forms of H+ ions present in the electrolyte or at the metal surface. This proton transfer is an electronless cathodic reaction which nevertheless achieves the charge balance in the global corrosion reaction: Femetal+++2Helectrolyte+→Fe++electrolyte+2Hmetal+This faradaic process thus forces H into the steel. It is no longer related to any inhibition of the recombination of Hads into H2 ads and can occur in parallel to hydrogen effusion.In a similar manner to Troiano's mechanism, involving the effect of triaxial tensile stresses, this kind of charging should generate high surface stresses capable of initiating cracks at the surface. Their combination with working stresses would then lead to a stress dependent cracking process. These assumptions are so far the only ones compatible with all the experimental data. However, they raise many basic questions, such as the role of pH, CO2 and HAc, the need for a new type of permeation test, and the relationship between the severity of stress dependent and non-stress dependent types of hydrogen cracking. The practical consequences could be considerable and new fundamental research appears mandatory.