Observation is made on the rf‐plasma oxidation process of a Pb alloy across the priorly existing thermal oxide. This is carried out through the analyses of the ultrathin (several nanometers) oxide formed by the oxygen isotope with the secondary ion mass spectroscopy. It shows that a good deal of oxygen arriving at the oxygen–oxide interface remains and forms oxides in the outer portion of the oxide. Hence the interstitial or substitutional transport of metal ions or substitutional transport of oxygen ions through the thermal oxide is more important than the oxygen‐ion interstitial transport that has been considered responsible for rf‐plasma oxidation. Furthermore, it is elucidated that the thermal oxide remains even after rf‐plasma oxidation except for a probable slight decrease due to sputtering in the early stages of rf‐plasma oxidation. This explains the importance of the thermal oxide in determining the properties of the oxide as a whole.