Austenite stainless steels (ASSs) may suffer from both cold deformation and sensitization prior to hydrogen exposure. There is scant data in literature on the combined effect of prior deformation and sensitization on the hydrogen embrittlement (HE) of ASSs. The present study investigated the combined effects of tensile plastic prestrain (PS) and 650 degrees C sensitization (ST) on the HE of 304 steel by hydrogen pre-charging and tensile testing. The results are explained by terms of pre-existing alpha' martensite content. PS higher than 10% can enhance HE significantly by inducing severe alpha' transformation prior to hydrogen exposure. Prior ST also enhances HE, but submitting the prestrained and alpha'-containing 304 steel to short-time ST can diminish the enhancement of HE by prestraining, as ST can cause the reversion of alpha' to austenite, reducing pre-existing alpha' content. It is inadvisable to make 304 steel be sensitized/welded firstly and deformed subsequently, even if the ST time is short such as what happens during welding, because this treating sequence can induce more alpha' than prestraining alone, enhancing HE more significantly. Apparent hydrogen diffusivity can be related quantitatively to pre-existing alpha' content, proving directly that alpha' platelets can act as diffusion "highways" in ASSs. It is indicated that pre-existing alpha' can enhance subsequently the HE of ASSs is because it can lead to a large amount of hydrogen entering the ASSs during hydrogen exposure by acting as diffusion "highways". HE is enhanced by increasing hydrogen amount rather than by pre-existing alpha' itself. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.