Herein, the first observation of the effective hydrogen diffusion coefficient of CoCrFeMnNi high-entropy alloy (HEA) was performed using electrochemical hydrogen permeation; further, it was compared with those of stainless steels (SS) 304 and 316L. HEA and SS 316L showed similar effective hydrogen diffusion coefficient of 1.75 x 10(-11) m(2)/s and 1.91 x 10(-11) m(2) /s, respectively. SS 304 showed the smallest that of 0.58 x 10(-11) m(2) /s in the study. Hydrogen diffusion through the grain boundary was dominant in face-centered cubic metals. Hydrogen permeation resulted in no change in the microstructure of HEA and SS 316L; however, it caused a martensitic transformation in SS 304. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.