Small nanoparticles of nickel oxide and nickel hydroxide were investigated for the oxygen evolution reaction (OER) in alkaline solutions. These nanoparticles showed excellent catalytic activity, giving a current density of 10 mA cm(-2) at overpotentials (eta) of 300 and 330 mV for Ni(OH)(2) and NiOx, respectively. The catalytic activity was comparable to that of the best mixed metal oxide catalysts under the same conditions. The activity was significantly higher than that of electrochemically formed Ni(OH)(2) and NiOx films, demonstrating the benefit of nanostructuring. Pretreatment of the working electrode was important for the activity and stability of the catalyst layer on the electrode.