The influence of electrochemically charged hydrogen (H) on the hardness (H-N) of a CoCrFeMnNi high-entropy alloy (HEA) was investigated with nanoindentation. Upon charging, H-N of HEA increases by similar to 60%, which decreases gradually during subsequent aging at room temperature, and on prolonged aging, the alloy softens to an extent that H-N falls below that of the uncharged HEA. These H-induced mechanical property variations are rationalized in terms of the competition between solid solution hardening caused by H and excess vacancy creation due to deeply trapped H. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.