We have made for the first time electron spin resonance (ESR) measurements on Si(111)7 X 7 and Si(100)2 X 1 surfaces during the initial oxidation processes at room temperature. The present results clearly show that, at a very initial stage of oxidation of Si surfaces where only a few surface layers were oxidized, a variety of defects are formed that have not been seen in ex situ ESR studies of SiO2/Si structures.