High Mn twinning-induced plasticity (TWIP) steels are attractive for high performance applications owing to their extraordinary ductility at a giga-graded tensile strength level. Hydrogen delayed fracture (HDF) came to the fore as a key issue to be solved for the application of these steels. Although it was found that Al addition improved the resistance to HDF, the reason was unclear. Therefore, in this study, the fracture surfaces of annealed and hydrogen-charged TWIP steels with different Al contents were examined after slow strain rate tensile tests. Diffusible hydrogen was measured by thermal desorption analysis. It found that the strong resistance to HDF was due to an alpha-Al2O3 layer formed below the (Fe0.8Mn0.2)O layer during the hydrogen charging in an aqueous solution prevented the hydrogen to permeate into specimens from the surface. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.