The electron spin resonance (ESR) observation of an isotropic signal of g = 2.00246 ± 0.000 03 in as-grown (111)Si/SiO 2 is reported. These structures were thermally grown in dry O 2 (≈1.1 atm) at temperatures T ox = 700−850° C, the maximum signal intensity being observed for T ox ≈ 800° C. The defect is characterized by a narrow symmetric signal of 1.0G peak-to-peak width and a 29Si hyperfine doublet of splitting a hf = 16.1 G. Etch back experiments have shown it to reside in the a-SiO 2 layer. Analysis of the salient ESR properties suggest that the originating defect is intrinsic to SiO 2, with the unpaired spin occupying an effectively highly s-like state.