This study investigated the effect of thermally induced surface martensite layer on hydrogen embrittlement of Fe-16Mn-0.4C-2Mo (wt.%) (16Mn) and Fe-25Mn-0.4C-2Mo (wt.%) (25Mn) steels through slow strain rate stress corrosion cracking testing and proof ring testing in wet H2S environment. The 16Mn steel had a surface layer of less than 150 mu m in depth containing epsilon-martensite, alpha'-martensite and austenitic twins. The martensite layer is found to reduce the hydrogen embrittlement resistance of the steel. In comparison, the 25Mn steel developed a full alpha'-martensite surface layer, which exhibited practically nil effect on the hydrogen embrittlement resistance of the steel. The epsilon-martensite provides much larger interface areas with the mechanical twins of the austenite in the 16Mn steel than the alpha'-martensite/austenite interfaces in the 25Mn steel. These interfaces are hydrogen trapping sites and are prone to initiate surface cracks, as observed in the scanning electron microscope. The formation of the cracks is attributed to hydrogen concentration at the e-martensite and austenitic twin interfaces, which accelerates material fracture. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.