ABSTRACTLayer-by-layer oxidation of Si(111) and (001) surfaces has been studied by using scanning reflection electron microscopy (SREM). We found that SREM images reveal interfacial structures of the SiO2/Si system. Our results showed that the initial step structure of Si substrates was preserved at SiO2/Si interfaces and that interfacial steps did not move laterally during oxidation. We also observed a periodic reversal of terrace contrast in SREM images during the initial oxidation of Si(001) surfaces. These results indicate layer-by-layer oxidation of Si surfaces, which is promoted by the nucleation of nanometer-scale oxide islands at SiO2/Si interfaces. In addition, we investigated the kinetics of initial layer-by-layer oxidation of Si(001) surfaces. We found that a barrierless oxidation of the first subsurface layer, as well as oxygen chemisorption onto the top layer, occur at room temperature. The energy barrier of the second-layer oxidation was found to be 0.3 eV. The initial oxidation kinetics are discussed based on first-principles calculations. Moreover, we confirmed that the layer-by-layer oxidation of Si surfaces holds true for conventional furnace oxidation.