In the present paper, in situ electrical and electrochemical measurements during oxidation of pure Ni in ultrasupercritical water (500–600 °C) have been performed and analysed. The onset of oxidation was followed using contact electric resistance measurements. The impedance spectra of the Ni–Ni contact during oxidation could be quantitatively interpreted using general considerations of the corrosion process and the Mixed-Conduction Model for oxide films. Preliminary estimates of the diffusion coefficients of principal ionic and electronic current carriers have been obtained and their relevance with respect to available data on Ni oxidation is discussed.