The initial stages of oxidation of the Si(100)-2×1 surface have been studied using STM and STS. In contrast to the Si(111)-7×7 surface, which has a metallic density of states (DOS), the dangling bonds of the Si dimers on the 2×1 surface are paired, leading to the formation of a surface gap and a vanishing DOS near EF. We observe a reduced reactivity of Si dimers towards O2 compared to that of Si adatoms on Si(111)-7x7, consistent with the reduced DOS near EF. Defects on the Si(100)-2×1 surface which have a metallic DOS dominate the reactivity towards O2 in the early stages of the reaction. Among the new sites generated by the exposure to O2 are 1.4 Å high bumps on top of the surface. Upon annealing of the O2-exposed surface or upon O2 exposure at an elevated temperature these bumps form elongated islands. Evidence is presented suggesting that the bumps and islands likely are due to silicon ejected to the surface by the oxidation reaction. Possible mechanisms and implications are discussed. © 1992.