It has been conjectured that atomic oxygen and ozone can have a great influence on the plasma oxidation of ultrathin aluminum for magnetic tunnel junctions. In order to measure the density of 0 and ozone, two-photon absorption laser induced fluorescence measurements are performed in the dc glow plasma that is used for the oxidation process. It was found that ozone is much more abundantly present compared to atomic oxygen. Using in situ, real-time ellipsometry measurements, we prove that ozone is not directly involved in the oxidation process. (C) 2004 American Vacuum Society.