Nickel cathodes exhibit significant deactivation during hydrogen evolution in alkaline water electrolysis. The addition of soluble V2O5 to the electrolyte was found to result in reactivation of nickel by formation of a vanadium-bearing deposit. The deactivation and reactivation phenomena were studied by impedance spectroscopy. The deactivation was not only manifested by an increase of the charge transfer resistance, but also by the emergence of an adsorption pseudocapacitance. Upon vanadium addition, the charge transfer resistance decreased, the pseudocapacitance was lost, but a constant phase element appeared. Most of the reactivation was due to the effect of deposited vanadium species, and only a little to the increase in surface area caused by the deposit. (C) 2002 The Electrochemical Society.