The structural degradation of thermal SiO2 on (100)Si under isochronal vacuum annealing in the range T-an=950 degreesC-1250 degreesC was monitored by electron spin resonance (ESR) in terms of point defect creation, including E-gamma('), E-delta('), EX and the elusive predominant degradation-center S. Depth profiling after heating at 1200 degreesC shows that the S centers predominantly reside near the oxide borders, generally in anticorrelation with the E-gamma(') distribution. The resulting anisotropic demagnetization effect has enabled inference of the S center susceptibility. As to the nature of the S center, an observed weak hyperfine structure may comply with the S center being of the type Si(n)O(3-n)equivalent toSi. either the single n=1 or a mix of both the n=1,2 variants. (C) 2002 American Institute of Physics.