Growing awareness of dangers related to NOX emission by industry and road transportation has resulted in increasing demand for sensors detecting NOX. An important class of these sensors use Pt-based electrodes applied on yttria-stabilized zirconia (YSZ) substrates. Performance of such sensors depends on redox reactions occurring at electrodes in the tested atmospheres. Impedance spectra and j-U curves of PtRh electrodes were measured in a 3-electrode mode, using Pt metal pads as counter and reference electrodes. The data were collected in a 10mHz- 100kHz frequency range at temperatures from 450 to 750°C. Each series of measurements was carried out in a gas mixture with different oxygen content ranging from 0 to 21 vol%. It was found out that the low-frequency part of the impedance spectra (characterizing electrode processes) was sensitive to the oxygen content in the gas mixtures.