Using a high resolution UHV compatible Kelvin probe we have performed work function (WF) transient analysis of metastable states on Si(111) 7 x 7 during the initial adsorption dynamics of molecular oxygen. The initial oxidation phase produces a large increase in work function of similar to 950 meV due to the relatively large charge transfer between the Si adatom reaction site and oxygen. Subsequent WF transients in the dose range up to 50 1 indicate the presence of metastable states decaying as the on-top oxygen is incorporated into the backbond, leaving the adatom site available for subsequent oxygen uptake. Exponential analysis of these transients produces lifetimes in the range of 8-22 min at 300 K, depending on the oxygen coverage.