In situ electrical and electrochemical measurements during oxidation of ferritic steel P91 and austenitic steel AISI 316L(NG), as well as of their main constituents (Fe, Cr, and Ni) in ultrasupercritical water (500 - 700 °C, 30 MPa) have been reproducibly performed. Features observed in those measurements were substantiated by ex situ results on the thickness, composition, and morphology of the formed oxide layers from weight gain measurements, scanning electron microscopic observations, and in-depth glow-discharge optical emission spectroscopic analyses of corrosion layers. The oxidation process was followed in situ by using the contact electric resistance and contact electric impedance techniques. Impedance spectra of the Ni-Ni and Cr-Cr contacts during oxidation have been reproducibly measured. They could be quantitatively interpreted by 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 and Cr oxidation is discussed.