How electrochemical capacitance is affected by the non-stoichiometric properties of nickel oxide (NiOx) films is reported. Nonstoichiometric NiOx thin-film electrodes were prepared for use in a supercapacitor by reactive radio-frequency (rf) sputtering of a nickel metal target. The oxygen partial pressure in the sputtering gas was systematically varied to fabricate various nonstoichiometric NiOx films. As the oxygen partial pressure increased in the sputtering gas, hole concentration increased from 4.7 x 10(16) to 1.0 x 10(18) cm(-3) while the specific capacitance of the NiOx films decreased from 148 to 80 F/g. A mechanism to explain the relationship between the specific capacitance and nonstoichiometry is proposed. The specific capacitance values of these NiOx films are determined by the number of Ni2+ states that can be oxidized to Ni3+ as a result of the double injection of OH- ions and holes. (C) 2004 The Electrochemical Society.