The Pt/YSZ interface was investigated at low oxygen partial pressure under high vacuum (HV) conditions at 400 A degrees C. Two different electrochemical techniques were coupled to mass spectrometric gas analysis using a new solid electrochemical mass spectrometric monitoring device. Under cathodic polarization, the lack of oxygen in the gas phase induces the reduction of the YSZ solid electrolyte which acts as oxygen source for the formation of O2- ions migrating to the anode. Under anodic polarization, both platinum oxidation and oxygen evolution reaction are identified. PtO (x) is formed at both the Pt/YSZ and the Pt/gas interface according to two different mechanisms. At the Pt/YSZ interface, PtO (x) formation is an electrochemical process following a parabolic growth law, while the presence of PtO (x) at the Pt/gas interface is related to the diffusion of PtO (x) formed at the triple phase boundary towards the Pt/gas interface. It is proposed that the side oxygen evolution reaction stabilizes thermodynamically the PtO (x) diffusion toward the gas exposed interface during the anodic polarization.