The metastable austenitic stainless steel is Fe-16Cr-10Ni, of which the grain size can be controlled between 1 and 21 mu m. Hydrogen precharging causes a critical drop in ductility during tensile tests for the largest grain size (21 mu m). In order to understand how efficient grain refinement is against hydrogen-induced ductility reduction, by varying the heat treatment conditions, it was possible to manufacture six different grain sizes and pinpoint the grain size at which the drop of ductility is critical. This change in ductility was associated with a transition from fully ductile fracture surface to a fracture surface composed of dimples, quasi-cleavages and intergranular fracture. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.