The presence of dissolved hydrogen sulfide in upstream or refining fluids is known to encourage hydrogen-induced mechanical failures of carbon or low alloy steel. Historically, the solution pH and the gaseous partial pressure of H2S (PH2S, bar) are used to evaluate an aqueous environment’s severity during materials corrosion cracking qualification. However, in recent years, it is suggested that the H2S fugacity (fH2S, an effective H2S partial pressure in bar) and actual dissolved H2S concentration ([H2S]aq) be used, rather than the PH2S, to better account for the effect of total pressure on steel corrosion cracking performance. In this paper, results obtained in a Joint Industry Programme (JIP) focused on evaluating the effect of fH2S on the hydrogen permeation across upstream steels using hydrogen pressure probe sensors and electrochemical Devanathan-Stachurski permeation tests are presented. A positive correlation between H-permeation rate at steady-state and environment fH2S or [H2Saq] is measured for numerous carbon steel grades in acetate-buffered 5% NaCl solution at pH 4 and PH2S ( 1bar), tested at total pressure between 1 – 300 bar. Furthermore, hydrogen induced cracking of a susceptible carbon steel grade increases when evaluated as a function of increasing fH2S for the same PH2S/pH combination.